Lower or higher LSA? Or does it have to do with something else.

 

A numerically lower LSA will make the engine "snappier", but narrows the power band.

 

solid roller cam revs the quickest

 

Rev faster in neutral from idle? or under load?

Very small overlap, which gives a smooth idle, helps low end and quick revving in neutral. Light weight rotating parts help a lot!

Nitro and a large blower seems to work well on TopFuel cars. They rev from maybe a 1800-2000 idle to 8000 in a couple tenths of a second.