Unless you absolutely cannot ever see yourself being able to get a new quad I would not put a cent into a Warrior. I assume since you want to increase the performance of your Warrior that you are wanting to compete better against other riders but you can throw thousands into a Warrior and barely have it hanging with a stock 400EX. If money is your main concern sell the Warrior and get a used 400EX... you can probably get a stock one easily around $3000.
I had a warrior a while back... I put at least $1000 into it and it was still a dog. When I sold it, I pretty much lost all the money I put into it. The 400EX is light years ahead of the Warrior and nearly the same price.
plan to race cross country or woods ride the warrior is great the best way to get power from it is a cam i had a white bros race series in mine you have to run heavy duty valve springs but any cam worth having is that way to get the performance from the cam you will need to port the head and a hi comp piston the porting is the most important because you will double the size of the passage ways and put an oil cooler on it verrrrrrrry important for reliabilty do not buy a 400 ex or z 400 keep the warrior and save your $ till you can get a 450the z400 and ex are just as outdated as the warrior when compared to a yfz :rock
well actually sooner yfz, i dont really have a problem keeping up with 400exs and all i have is my k and n and a pipe and sum other mods and it was pretty easy keep up actually, maybe u jus exaggerated a lil. but i am trying to sell it, it is in the for sale section