Hello Tim,
Certainly within Hauptwerk itself, Launchpads (or any other MIDI devices) have negligible RAM and CPU overheads (a small number of KB). I wouldn't expect the drivers themselves to have significant additional overheads.
Note also that Hauptwerk has full native support for the original 'mark 1' Launchpad models (which are readily and inexpensively available second-hand), but not yet for the 'mark 2' models, such as the Launchpad Pro:
viewtopic.php?f=4&t=15870&p=119317#p119316