GM did a lot of trick with the new one.
What Bats is saying below
Exactly. The TT 911 and the GTR aren't all that great for MPG.
What GM did right with the LSx was to hone the combustion chamber, up the compression ratio and properly mate it to a proper 6 speed manual tranny that at 65MPH you are between idle and 2k RPMs.
When you have so much low end TQ you can take advantage of that by having the car cruise at freeway speeds in the last gear and sip gas.
- - - Updated - - -
I beg to differ. I build high hp turbocharger vehicles everyday that exceed MPG of most hybrids nowadays. High compression, a good combustion chamber design (basically an engine with an incredible VE) and a little boost goes a long way; especially allowing a nice lean burn without detrimental effect to the engine. What should be considered when building a high HP small displacement engine is getting the engine to be as efficient as possible before the boost is introduced. It sounds crazy to some but it works..extremely well. The average 500 WHP 1.6 liter setup I build gets 38-44MPG depending on off boost conditions vs boost conditions and/or gear ratios. The average 600WHP 1.9-2.0 liter setup gets 34-38MPG. All of these setups will have no less than 6k rpm of usable power band which is more than most offerings today. My CRX made 638WHP on 93 octane and it has a 10.5:1 compression ratio. Because of the aggressive nature of the cams it only get 28mpg, but it also makes over 1000BHP with the right type of fuel and more boost so it is a compromise I can live with. The last 5 speed turbo NSX I tuned is averaging 30-32MPG with a best highway MPG of 35. Granted, the type of work I do to achieve these figures is not what the OEM wants to do, but if they really wanted to make something that performed and got great mileage they would do more than just give it more selectable gears and/or longer gear ratios.
A V8 and up is the worst for fuel economy based off its sheer displacement, and rotational/frictional losses on its own and I won't even get into how terrible the cylinder heads are on the domestic side of things. Even if they turn off cylinders its still just dead weight being thrown around that the other cylinders have to work to turn. The only thing a corvette has going for it is ridiculously long gear ratios just to keep it at near ilde when cruising on the highway. It still has a terrible VE; a very inefficient engine although it's greatly improved over the years.......
I should have clarified that what I state is what OEM are delivering to the consumers. Because that is what I'm talking about with my GTO, for example. A fully loaded 3800lbs~ with the additional weight and load of a Maggie blower running 91 octane California Costco gasoline.
Let's line item your projects:
500whp 1.6 liters getting 38-44MPG
600WHP 1.9-2.0 34-38MPG
CRX 638WHP on 93 octane with 10.5:1
I like to know the following:
Will these cars pass the California sniffer?
Are the cars with good MPGs gutted interiors (in other words have the cars been through a weight reduction diet)?
Do the cars have modern day safety standards (airbags, etc.)?
Any of these cars running pump gas with no meth or water injection?
Explain to me just how bad are the Corvette heads. They flow well, make good power and torque. The cats are not much bigger than the length of my hand (leading me to think that they don't pollute that much since they look like pre-cats).
Sure they have enough power to support their near idle freeway cruise. That is really part of my point of having displacement and using those attributes.
- - - Updated - - -
Japan hasnt been building turbos instead of V8s for years because they did not improve gas mileage vs bigger engines/more cylinders, have equivalent power when wanted, keep overall weight of vehicle down (meaning less JCI fees every year, less toll road fees, etc), proven reliability.
When I looked into converting a FD from a rotary engine to a LSx I discovered that since the LSx was an aluminum block it was pretty close in weight to the Japanese small engine + turbos + whatever needs to support the turbos (Intercooler, additional wires, hoses, ducts, etc.).
I have a loaner 2013 Acura TL and I get almost the exact same average MPGs as my GTO. Both roughly around 20MPGs~.
I drive both cars hard. Both are probably about the same in weight. And I think both have 6 speed tranny (TL being the auto).
The problem with modern car weight (which is getting heavier due to increased size and all the safety, infotainment stuff) is that you need TQ to move it.
So while the import cars are getting heavier whatever advantages in a smaller engine is starting to get negated since I have to mash on the gas pedal longer to get it to the speeds that I want to hit before I cruise.
With the GTO: WOOOOOT! and in a short amount of time I'm at the speeds that I need to be and go into cruise mode.
I just think there is a better way to MPG and it certainly isn't a bigger engine. 
