Now that my unbeliveably advanced brain has had some time to think about this, this might turn out to GCs advantage, you see, the Xbox version of this pack is released 4th of November, day before True Crimes, so there is a chance that Xbox owners will buy GTA, which makes True Crimes sales lower, which gives Cube a chance for better sales than Xbox version.
or that very few people are going to get GTA, when its old, and most have probly played it on PC and/or PS2 already.
So I see 3 possible scenarios, 2 good, one bad, bad being that both True crimes and GTA sells really well on Xbox, good ones being that R* gets pissed at MS due to low sales, other is taht Activision gets pissed do to low sales of TC. Fingers crossed.