Okay I was looking around for framerate fixes online for this (pc version obviously) and the one that worked for me was manually turning off crossfire, it just doesn't work in this game.
Course since it's a 360 / ps3 release, a single 5870 is more than enough to run it at almost a rock solid 60 fps with everything turned up (the few options there are).
I've now played each act in the first 6 zones as well as doing the metal sonic rival battle and the battle with sonic 2's final boss.
My thoughts are so far- modern sonic versions of classic sonic levels are OK but nothing amazing, classic sonic in classic stages are for the most part good outside of a couple sections in each. Modern sonic in modern stages however is pretty fun , classic sonic in modern stages slightly less so. I would have really liked if they'd simply made a full on classic sonic game with this engine, 2-3 acts per zone with a proper boss for each , each stage brand new and custom built just for this but I can see why they had to mix the 2 together.
The first boss fight was kind of annoying , they tried to add a quicktime event and change it up from how it worked originally , only annoying because I didn't instantly figure out how to land on the arm.
Rival battle with metal sonic was fun though.
Once I fixed the preformance issues and got to playing it a bit more I'm finding this game is pretty good overall. I Still have to get the keys for the second boss, do the second rival battle and open up the last few stages but this is already more entertaining than 90% of the sonics that came before it.