Voice-chat will have a negligible effect mate. Don't try and dress it as though it would. It's like with CoD, it had Theatre Mode in Ghosts but it was absent in Ghosts, when quizzed the developers said they didn't want it effecting the bandwidth as they didn't feel the feature gave enough to the game to compensate for that downside.
MKTV will effect the bandwidth, it's not just capturing the sat logged from the game, it's capturing the data transmitted from other people's games into yours and so will be a drain on the bandwidth in some respect... more so than voice-chat would be!
Unless you can only use MKTV for offline races, then it's a bit of a meh feature.