Yeah, it would be doable as DLC but they will make more sales with a disc-game because not everyone that bought the game had XBL so your base shrinks when not everyone can even get it.
XBL is supposed to have already launched its "download full games" service by then so maybe they'll release it that way as well as a disc version.
The question then becomes... how do you pay for it? CC? MS Points?
But a big drum kit can definitely add dynamics to a song. He doesn't want a Portnoy sized one because it's unnecessary, but he doesn't necessarily want a six piece kit to work with. We play progressive rock, so with all the changes and stuff, it's definitely a plus to have a decent sized drum kit.
