Flatter isn't always better here boys. If you're getting 3000fps out of a 6mm 105 and 2750 out of a 6.5mm 140, it's going to be a wash in wind....which is what really matters. The 6.5 will yield longer barrel life and lower pressures which would likely make it less finicky in a gas gun.
Don't get me wrong though, I am a big fan of 6mm and have seen some pretty impressive results from the 6mm gas guns. Good luck with the project Doug, you know you can always give me a buzz if you need anything.