You can absolutely spin the tire from not enough SLR if you are not planting the tire.

That said, I find it hard to be convinced that a small step like that gear change is could make a car go from good to junk. Assuming a 3 speed, that's around a half point of gear ratio. Go back to the old set up and see.

I can tell you that running on a track that is prepped for fuel cars, along with the clutch dust and oil they leave behind, is not typically going to be the best situation for 9 and 10 second door cars.