There were many slowdowns with GBA games with the supercard in the past, but those have been reduced.
The only game I can think of that is completely incompatible with the supercard is Advance Wars 2: Black Hole Rising (any version). Everythign else seems to work ok.
If you continue to encounter slowdowns in a game you can easily disable the 'real time save' function in the rom, and that will eliminate any slowdown (removing real time save will not effect acctual save, which must be saved using the Start+L+R+A function, for GBA games that is).
Check out scdev.org (the unofficial supercard website) or supercard.cn/eng/index.htm (the official supercard website) for more info if you like.
And yes, both the supercard and the superpass2 can be upgraded, although I think there's only been one upgrade on the superpass2, showing just how simple the technology is.
Also, both supercard products have been found to work with every DS, including the latest Lite version.
There are little to no DS incompatibility issues with the supercard and superpass2.
Hope that helps.