I'm actually really glad to see a topic about this -- I think it's kind of frustrating when people sum up the R4's compatibility as "everything works 100% except for some freezing in PoR if you're not using a Toshiba card". For those of us that aren't really into Castlevania games (but do enjoy hurling our eSelves downhill on wheels at remarkable speeds whoa-whoa-whoa!!!), it's sad to be told that compatibility is perfect when Downhill Jam's choppier than a vegetable-slicing circus.
I'm inclined to side with those of you who say that Downhill Jam's issues are not necessarily related to microSD speeds, but issues that will hopefully be corrected in later iterations of flashcart firmware. Sk8land works 100% on the R4, and from a technical standpoint it's pretty much the same song and dance -- 3D cel-shaded graphics with streaming audio. Additionally, while Downhill Jam plays much cleaner on my G6 Lite (which is incidentally where I play it from all the time), there are still a few little framerate hitches that not even the fixed-capacity awesomeness that is the G-fucking-6 could correct.
You can certainly see the difference when playing it on the legal game card. And I would just play it on the legal card, but I love the R4 too much -- and any game card that usurps that the R4's resting place (read: inside my DS) is a game card that won't be seeing much use.
Hopefully they get a fix together for it sometime soon. I'd like to play it from my R4DS without issue.