I had emailed Powroll way back when then 250F was just coming out and they replied to me and said they were hot on the trail to modify it. Well as you can see, they haven't done much for it, which means to me that it either can't be done, or isn't worth the cost or has a reliability issue. I was hoping for the same kind of thing as well...I ride the Vet class and a few more beans wouldn't hurt!
Yamaha's own engineer said this engine was desgined for 250cc, and increasing the displacement would not be practical. I would have to believe that this engine is not "over-designed" and pumping more horsepower through the crank and transmission would seriously affect reliability. I'm with you on wanting more power, but I'm waiting for Honda/Yamaha to make a big-bore that's no more than 240 lbs on the starting line. There's no way I'll give up my 250F until that happens. :D
The 240 pounds should happen soon. Without any evidence, I believe that lowering the weight of the 426 or 450 is one of Yamaha's main goals. Heck I think they could just drop 10 pounds off the current 426 and have a winner. To do this they will probably be putting out a new motor and that means it will probably be a 450. I would bet that the new Yamaha will kill the Honda. Of course I am not biased though.:)
I'm hoping Yamaha does exactly that. Given my experience with Yamaha's 4-stroke packages, suspension, handling, and reliability, I will own a 2002 YZ450F if they manage to shave at least 10 lbs off it! I'm keeping the 250F no matter what. Out of all the bikes I've owned througout my life, this bike is absolutely THE most fun I've ever had on two wheels. It would be sacrilegious to turn be beloved little piglet over to some zit-infested spode-racer who would surely subject it to a slow, tortuous demise.
The gentleman I spoke with about a month ago at Thumper Racing said the sleeved 280cc YZF would run right with his new KTM400. If it proves to be a durable mod. they can sign me up for one. He also stated that 280ccs was the max size they were comfortable with.