Hi there,
I had the same concerns as you a couple of weeks ago and somebody on another forum said that through flight1 it is possible for a refund within 30 days (perhaps someone can confirm that "low fps" is a valid reason for a refund, but anyway...) so I took the plunge.
I have an M11x R2 (i7 processor) and it runs on fs9 (stock scenery) at a good 50+fps (2d cockpit). When I'm in the air in the vc it can drop to 15fps in clouds. I also installed uk2000 stansted extreme scenery and after locking the fps at 26 the ifly still stays there in the 2d cockpit (I haven't seen the 3d figures yet but I'm mostly in the 2d anyway).
I had just usable fps in fsx and I'm glad and surprised it worked out so well for me in fs9 given that the M11x isn't the worlds best simming computer, bless it's little cotton socks.
Good luck whichever way you choose to go!
G