Rice#67 - The YZ144 is actually a fun and quite fast bike to ride. Stock I had to add a couple teeth to the rear sprocket to get up some steep hills without the motor losing steam. This worked well, but I was shifting quite a bit. After putting in the 144 I was able to put on a sprocket with one less tooth and the bike continued to pull great. I run a stock pipe with V-Force reeds and mix 32:1 with Honda HP2 and a 50:50 mix of 93 octane and race fuel.
The one thing to watch out for is piston life. I know with engine work and increased performance you will have to replace the piston more often. The exhaust port bridge is pretty thin on my cylinder so the piston wants to start to groove and you'll hear a high pitch squeal. Eric is very good about working with you on this though, so mention you want longer life on the piston and he can leave the exhaust port on the thicker side when porting.
I love the bike, even though I haven't ridden much in the past couple years, it's a fun machine to ride. I know my friends love the bike even coming off 450's. Their is something pretty great about a fast lightweight bike! Let me know if you have any other questions.
350F's
Yamaha - came out with 400
KTM - has so many different engine sizes
Honda - came out with the 150F
I wouldn't be surprised if one of the 3 above was the first to come to market with a 350F. If I was a betting man... I'd have to agree with "flynbryan" and say KTM will probably be first. They appear to target that nitch market, and I'm sure a 350cc engine would make a great woods bike. They came out with a stock 144 fairly quick, so they can probably pull out a 350 the fastest. It may not be the best, but I'd say they could be to market the quickest. Honda or Yamaha would probably be second as they seem to design from the ground up so odds are they may not be first, but they may be better.