What's your GPA? An SMP is risky. It's best if your GPA is low to the point where you are in too deep, and can't salvage it with post-bacc classes (like <3.2). Like apumic said, if you don't rock the SMP, you have pretty much screwed your chances at getting to a US school, but if you rock it, you will prove to schools you can handle a medical school curriculum. If you have something like a 3.4+, you'd be crazy to do an SMP.
I think it's hard to say whether someone usually does better in an SMP than in undergrad, because it varies on the program and individuals work ethic. Georgetown's SMP probably has a higher percent of people who do well and has a proven track record of success compared to, say, Podunk state. I'd say anyone who got into med school after an SMP did better in it than in UG, though.