You will probably get a faster time with smaller inch rims..
Im no expert either..
but i remember when i was at the drags, a guy from this forums put his stockies on and got a much better time than his 18 inch rims lol...
Anyways i dont think you will find some difference between them.. It will come down to your launch, and how well the rubbers grip..
Do the 255s, have better rubber than 245s??