I dont' know about the YZ, but if you don't re-jet when you get a pipe, you wont notice much. But, every bike that I have put a pipe on(and re-jetted) has responded big time. I usually go Pro Circuit.
On my '92 RM 125 much more mid and top, much more fun to ride.
On my '94 CR 125 smoother, fatter midrange and more top.
On my '95 CR 250 more everywhere.
On my '02 RM 250 night-and-day difference, totally different bike.
If your bike is currently jetted right, is well maintained and you want more power, get a new pipe. I have never been dissatisfied w/ FMF or Pro Circuit, but perfer Pro Circuit. $160(for a pipe) is $15 more than you'd pay for a reed valve and I think it makes a bigger difference. V-force reed valves and aftermarket pipes do work well together, though.
I've never seen porting and head mods done for less than $200 and they're permanent. If you don't like your pipe, take it off and sell it to a friend. If you don't like the porting, good luck.
They'll tell you that it's all in my head. But, I'll tell you that going from mid-pack starts to getting hole-shots has me sold on aftermarket pipes. THEY ARE WORTH THE $! :flame: