Ben,
Usually folding the gear will give you 15 knots or so - Compare other airplane model pairs where that is the only substantial difference (Cherokee 180 vs. Arrow I, Cherokee Six-300 vs. Lance, 172/182 vs. their RG models, Sundowner vs. Sierra, etc.) and that's about what you get.
In the case of the Cirrus SR22T, I think two things that haven't been mentioned yet would reduce that difference. First, their landing gear is pretty low-drag compared to the other models I mentioned. Second, you're talking about cruise speeds at altitude in a turbocharged plane - Up there, the air is thinner so the drag of the gear will have less of an effect.
So, I would guess that retracting the gear on an SR22T would only buy you 5-10 knots at altitude and that the added weight would cancel out any advantage you had from lower drag in the climb, so that you'd effectively have no difference between the two in the real world.
Let's say that you cruise at 25,000 feet and you're taking off/landing near sea level - You're going to spend 75 minutes climbing/descending (assuming average 1000fpm climb and 500fpm descent), so it's doubtful you'd spend more than 3 hours in cruise. On a trip of that particular distance, the difference would be minimal: If you figure 210 knots on the fixed-leg version and 220 on the folding-leg version at the same power setting, the 3-hour cruise in the fixed-leg version would get you 620nm. In the retract version, that'd take 2 hours, 49 minutes, 5 seconds - Yup, for all the extra money you'll spend in insurance and maintenance, you saved 10 minutes on a 4.25-hour trip to be able to say you went faster.
The thought is cool, though... But if you want to go fast, forget the folding-leg Cirrus and go right to the Mooney Acclaim Type S. 242 KTAS at altitude, and folding legs. I sure wish I could fly one of those - I'm really liking the Mooney Ovation at 170 KTAS down low!
