The softness is likely to be the cells, which you can compensate with a faster motor (ah the beauty of brushless), but if you're getting new cells I wouldn't bothed.
That said if you want to keep running the old cells all winter then a faster motor would compensate. I did this at this years York regional. I would normally run a 6.5L but the straight was long and I was on last years cells (keeping new ones for nationals) so ran a 5.5L. It got hotter than a 6.5L with new cells would have been but still fine for use.
I think the 4.5 would be too much at Worksop becasue there is no drag, but it's your money
