7.8:1 is pretty low already Adam. You also need to decide where you will be using the car, ie road, track, strip etc.
If you want it for a road car the 76R will be pretty laggy on a 2.1. I personally wouldnt lower the CR anymore as that will make the driveability even worse. Just make sure you run good quality fuel, maybe add some methanol. Also make sure its mapped well.
A 3076R with a .82 housing is a genuine 500+bhp turbo, if youre not aiming for that then i would stick with the 3071R. The 71 will top out at approx 450bhp but obviously the spool will be better.
If i were you id decide what you want from the car and then spec a turbo to suit. Maybe a GT2871R will meet your needs?