Escorts have small tunnels in them and 5spd sierras fit in them. And they will handle the power. A stock SR20 is only about 140hp, maybe an S13 one is a bit more, but no more than 150-160hp I'd guess. Less than a really hot 2L OHC Ford anyway.
If it ever was a problem (which I doubt) Quaife make gearsets for the type 9 which are rated at 190 and 270hp.
So don't cut it up!!! There's always more than one way to skin a cat...
also with the battery tray, what I did with my zed was drill out the spot welds holding it in, to get it out to paint it with PA10 and get the top coats right in and under and behind the tray, and put it back in with small cad plated bolts. No one has ever noticed.
So you could bolt the tray back in your car later if you wanted. Cutting the radiator support is reversible by replacing the whole panel again later on, but I would try not to do that. The alloy radiators on new cars are so thin you might be able to fit one in.
Cutting the radiator support panal and cutting the guards to fit flares are the two things I regret doing to my car. And mine is just a rusty old coupe!