^ I did a long series of analysis on my rear end ratio options a while back with math and graphs, just to confirm basic intuitions and whatnot: I basically ended up deciding that on paper in terms of power under the curve averaged across all reasonable track speeds, you always want to go to the biggest rear ratio possible unless you're running out of top speed in top gear. The fundamental reason for that is that the ratios between gears in the stock transmissions get smaller the further you shift up.
The only real mitigating factors are these: (1) You'll shift more often. If your shifts cost significant time (and they do unless you have some amazing formula-car electronic sequential), you could lose time in the net and (2) The higher gears might be weaker in a given street-car transmission and break earlier if spend so much time in them. I think especially on the 7AT that's a very valid concern, and that's what keeps me from trying something insane like a 4.9 (it exists!).
In an ideal world, of course, you'd just swap the ratios in the transmission itself so that they're all closer and optimal (and you'd tune ratios to each track you ran).
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