I've ran mid-grade and regular, both work fine. This isn't an old turbo, the ECU should be protecting the car from knock. Also, I believe (but could be wrong) that cars for sale in the US are required to be able to run on 87 octane. That being said I know tuned BMW's (M3's) run like crap on regular, but that's a totally different setup. Give this engine lower octane and it should just turn down the boost, like it's doing every 1st & 2nd gear anyways. Can't comment much on fuel mileage, as all grades of fuel here are laced with 10% ethanol all winter long, because one @#%$@ county requires it (not mine, but fuel distributors don't care), so it all sucks. Regular may be slightly less than 92 but not enough to be really noticeable or overcome the cost differential.
Come to think of it the 10% ethanol may be helping the knock/boost situation because ethanol has a very high octane rating (one positive to offset it's low fuel density). Either way I only put it in when I know I gotta burn a whole tank stuck in traffic or going slow, I'm feeling poor (which happens a lot these days) and the differential is in excess of 40c/G. Premium is now over $4/G here.