Mostly emissions and mpg, and the fact that the LS2 is out of production for domestic car service. You can find a LS2 with 390 hp in a Trailblazer or Saab 9-7, and several instances of a VVT edition of the L76, but in cars, the G8 is the only North American GM product with 6.0l displacement, and is largest outside of Corvette. The L76 in the G8 is the only L76 with AFM (active fuel management) designed to power down 2 or 4 cylinders under low load to return higher mpg. The hope also is that however the engine is configured when the manual arrives, that the dreaded Gas Guzzler Tax will not appear on your window sticker. ON that note, right now it is rumored/stated/assumed that the L76 with Tremec 6-speed will not have AFM.Curious as to why the General didn't use the 400hp version from the GTO in the G8?
Oh, almost forgot. Tunes can actually get you more MPGs. It is all how you use the noisy pedal. I have found that in my old WRX, that I had an off the shelf tune on, I had about 20 more horses and I could use them for good and squeeze a mpg or 2 out here and there, but having the power available, and not using it, seemed like such a waste. HAHA!And how would this affect gas mileage when NOT in WOT.