I'VE ALSO NOTICED:
The 400EXs seem to pull a lot harder and spin the tires much more in the corners. Because of this the 400EXs slide in and out of the turns much smoother and quicker than us 250R riders.

 

I don't buy into the 400EX is easier and less tiring to ride idea. First of all, if you race, then physical conditioning should be very important to you. Second is the fact that the 250R makes smooth, seemless power. If you send your cylinder off to some hack, you will be disappointed with the end result. You don't need a wild port job or even huge cubic inches to be competative anywhere except in the dunes or a drag race. My 85 ATC has a FMF pipe and a top end that desperately needs replaced, and it will run with a piped Banshee. My 88 250R is all stock and slow in comparison, but is very fast on the trails, and not in any way tiring to ride. It has a "lazy" kind of power to it. With proper bolt on selections, it will be quite a bit faster and still have that smooth power delivery that the R motors are famous for.

 

Take it from someone who owns both & comes from a long background of both 2 and 4 stroke bikes, although the 250R has very nice power delivery for a 2 stroke, it cannot compete with the smoothness of a 4 stroke. The 4 stroke will not deliver the punch of the 2 stroke, but it gets more of the power to the ground, easier.

Before I bought my 400EX, I spend many days at a private practice track, we traded off between my ATC250R (FMF pipe, .040 over, durablue axle, & freshly built suspension), my friend's stock '99 400EX, a couple of (basically stock) Banshee's, and another friend's '88 TRX250R (Works suspension setup for MX, FMF pipe & ram valve, 18" rear tires, & freshly built engine). Consistantly I could turn the fastest lap times (of anyone riding any bike) on the 400EX, even though the 250R was NOTICABLY faster. These were timed with a stopwatch with only one guy on the track at a time. The EX took less work and was more forgiving than even the 250R. I could endlessly turn fast times on the EX, where I could only turn several on the R. Keeping in mind at the time my personal ride of choice was the ATC250R.

If you were going for an all out custom MX bike the 250R would probably be your best basis, but for real world bikes with little modification, the EX is a very competitive choice.

At the Missouri Harescramble series, the 4 stroke C class (dominated by 400EXs) winner, many times, will have a faster overall time than the top 2 stroke B (dominated by 250Rs) rider. At our local MX track, many times the 400EX's take top positions, but so do the 250R's, just depends on who's riding them.

If I were given the choice today, of a brand new 2001 400EX vs a brand new 2001 250R, I would probably choose the 250R. But, I don't have that choice, and there is a whole lot to be said for a brand new bike that is very competitive with the old 250R.

The bottom line is, you can be very competitive with either bike, the 250R probably has more potential on the extreme end of the modifications, but the 400EX can be purchased new and many OEM parts are extremely cheap (being manufacured in the US). I did the testing, made my choice, and have done well in racing so far.

 

I am completely in line with your train of thought, however, there are a few points I disagree on and it could be because of my specific background.
The more a machine is modified, the more you target it to perform in a certain way. For instance, your ice racer is going to do terrible in the mud bog. Extreme example? Yes, but look at engines, to make more horsepower, you have to turn more revs, at the expense of low end power. so it seems to me, that a stock motor "R" varity, that has just a few mods bolted on to help it breath would be perfect. A little more mods to target your specific riding style or area and you are happier than a pig in, well you get the idea.
As far as a harder power hit being tiring, not in my book. I have been riding open class bikes since the days of twin shocks and air cooled cylinders. For me, I learned throttle control at a very early age. Also, I am 210 lbs so that also helps tame the "hit". I like my motor to have a good, strong, midrange hit with a good top end and a little over-rev, just in case I need it. I like to ride the midrange and just a little on the top end.
About twist throttles, I found that the stock spring in my carb, which is light for the thumb throttle, delevers too little resistance for a twist throttle. The machine is very hard to keep at a setting. Tomorrow I am going to match up a spring from a CR and see if that helps.
And while I am on my soap box, let me give my two cents about powervalves. Before the valve, you had to target a specific RPM range that you wanted your motor to run, and port/modify accordingly. What the valve allows you to do(and the factories have done a great job right out of the box) is let your motor make max ponies accross the entire rpm curve. The end result is a machine that awesome power everywhere. The fastest bike I ever owned was my KX500, but it made lazy power. By that I mean it was easy to ride, more throttle equals more speed. No suprises in the powerband, if you whack the throttle open, you go right NOW! The hardest hitting bike I ever owned was a 79 RM 400. You better be hanging on and having your weight foward. When that bike came on the pipe, it came on the pipe, brother. That was because it was air cooled and no valve. It was a bike designed to go from corner to corner and across terrain with no fuss and no muss. Anyhow, sorry for the long post, probably ain't nobody gonna read it anyhow. Later.

 

ridzhard,

When people say the R is more tiring than the EX, I think its a number of factors, only one of which is the hard hitting powerband as opposed to the power from the bottom of the EX. I think that with the amount of clutch work, throttle, etc required to keep the R in just the right gear you do get tired much easier on it than an EX. I just traded my 300ex for a 250r and the R is much more difficult to ride. With the Ex you just put it in gear and drove, you could just torque out of the corners and not really worry much about gearing, while the R has to be in the perfect gear to come fast outta the corner. For me, its a good trade off, because the R is just a much better machine. I havent owned a 400 so I dont know about it.