My car has the Mountune hoses and pipe, intercooler, and 3 pass radiator. So Heat Soak is NOT an issue. I'm also running the FACTORY thermostat. I do NOT have any tune installed, that is 100% factory. With the all season tires I run in the summer it's traction limited in 1st, 2nd, and some of 3rd on certain pavements so I just don't see a need for more power.
Note, in cold or cooler weather I do see about a 5 mpg DROP in fuel MPG while the engine is still cold. This thinking a colder thermostat should re-think that approach. Because that is simply a bandaid for an inadequate cooling system. Increase the radiator capacity and you'll not only see a gain in MPG's but you will also see a gain in power. This engine runs fantastic at 195 degrees and below or above that temperature there is a loss in economy and power. The Mountune 3 pass radiator allows the thermostat to maintain that temperature even when you are climbing a 7.5% 2 1/2 mile grade in I-64 heading into West Virginia in mid August.
So, here is what I am seeing for mileage. On level toll roads at 74 mph I will see 39 mpg. Driving on I-77 in WV I will see 34-36 mpg. Seems that curves, hills, and morons will impact fuel economy. In city driving my last fill up computed out to 33.2 mpg with an average speed of 29 mph. Note, tattletail was indicating 34.9 mpg, which is typical. I'll also not I only run 93 octane, feed this car 89 octane and it will cost nearly 5 mpg.