I am thinking of buying one today and was wondering if it would help much with performance?

It should help a little. Do you have any other mods which improve flow through the engine, such as a pipe? If so, you might want to look at re-jetting as well.

No, I don't have any other mods. This would be the first. Would it be a good first? Also, I don't know much about jetting, can you explain or point me to something that will explain?

i wouldnt get the k & n. i think they are good flowing but they also let dirt it along with air

I don't want to start an argument, but I think the UNI filter is better especially in dusty situations. Yes the K&N will flow a little better, but the UNI is easier to maintain (just my opinion). To answer your question any air filter that is better than stock is a good thing to have. Don't expect big power by just changing it though.

Jetting tips:

Carb tuning
Keihn carbs
Carb theory
More info

K&N website
Good place to look up the part number of your filter and shop around for the best price. Get an outerwear for it and it will go through about anything save water.

Another thing to remember is a 300ex only has two little frame tubes supplying air to the box, without some lid mods,or another snorkel added, a good filter will only flow what the box can get to it, even with the stock filter, open up the air box a bit,and you will see a bigger difference than a stock box with a performance filter. Just don't forget to rejet, more air will lean you out and drop power,and possibly hurt the motor if it gets too hot.