You have to remember that Razr come in both MX and XC versions. Maybe that person had the 2 or 4-ply instead of the 6 ply, in dirt the 6-ply work good but in sand I know nothing. Watch out for which FMF exhaust that you get. One of the mags tested one FMF style and said that it was junk. The Sparks full system and the WB/E series was rated the best in almost every test. Just have your rear shock redone, the rear one just needs a little work like on the rear of a 250R. On fronts shocks you get what you pay for. It is better to get best to begin with.
ATVSport.com has a test of exhausts for a 400 with dyno's, but you have to do some looking for it.

 

the maxis razr's will get chewed up riding desert. a friend of mine wore the equivalent of one season of trail riding off his razr's over a three day weekend in the deserts of eastern Oregon. I'd go with something designed for desert like the Kenda Desert Dog (the name is corny but they really are good) or the Maxis Bandits. If your stock tires aren't to worn then you might want to stick with those.
atvwheelie is right about the curtis sparks pipe. it kicks @$$ and has the shootout wins to prove it.
the shocks you mentioned are an awsome deal but if you have a mind for saving a couple hundred bucks have your stock rear shock revalved buy a Race Tech technician. unless you're riding REALLY hard you probably won't notice. anyways, it's money that could be going elsewere(the engine).
have fun.

 

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he means pure sports bandits, not maxxis.