Big bore kit

I had a big bore kit on my 2000 YZ125 and it was amazing.  I know Athena makes high quality stuff.  You may want to talk to Steve Landsberg.  Landsberg has had many YZ250F big bores, currently a KTM 350 fan.

The old saying is there's no replacement for displacement. I have seen folks have good results and bad results with big bore kits.
I still think the best money spent on a bike is suspension that works for the racer, ergonomics for the proper ride feel and fresh tires.
As far as the engine goes, the most important part of any engine modified or not is to stick with a scheduled maintenance interval. A pro with a 250f will want a fresh top end every 10 hours or so. An Old timer vet could get by at 80 hours. I might reduce what ever interval I was using with an OEM top end by about 20% with a big bore kit.

Hope this helps