Even with a 100 (or more) less power than before I STILL have trouble putting the power to the ground. Spinning your wheels is fun and all that, but it sucks when your actually trying to get somewhere; like to the end of a 1/4 mile or coming out of a corner on the track!
MPG is important if you have a small gas tank, it has a dramatic impact on your cruising range miles. It's not my first consideration, but I won't ignore it either. Getting the compression down so I could run pump gas was a good call. Having to run race gas get's old in a hurry, it aint easy to find and cost an arm and leg to boot.
I suppose Shelby, or a lot of other people, "would have", "could have", "should have" as the years roll by, technology marches on. But they didn't, they made the best call for what was available at the time. As I recall it, Shelby's FIRST engine choice was a 800 pound Olds V8 (he always liked Oldsmobile, go figure). The 289 was an extreme light weight in comparison, even the FE was a light weight. Small block Chevy, at the time, weighed even more!