I'm not sure why the Sirrus 6.0 is $3,000. It better be a 15 pound bike, which I highly doubt it is.
On paper the FX 6 is the faster bike. The lower stack and longer reach will get you more aero and better leveraging.
https://99spokes.com/compare?bikes=t...20;*z.MD|w.700
I'd save the $800 and use it to buy better accessories: Saddle, grips, inner barends, clipless pedals, cleats, longer stem, etc.
For reference I'm averaging 20+ mph and keeping up with A Group Roadies on my FX.