what peltor/3m call a 'kit', most of the world appears to refer to as just 'headset' - just the speakers, mic, and optionally earcups.
At this point, probably going to skip the earcups because it looks as though it'd require significant modification to my helmet to fit them - which strikes me as 'probably not legal, maybe not safe'.
Terraphone stuff is available here, but based on its reputation (mainly the clubman stuff I'd guess), I'll pass on that.
The Terraphone Pro Plus stuff is peltor compatible, but the open face headset ($105 from rallylights.com) is basically the same price as the comparable peltor (MT632, $110 from apdmro).
Seeing as I'll be picking this stuff up initially to co-drive (and not with any particular car), it makes sense to me to go with something that's a known quantity here - while I'm sure it's possible to make a OMP Clubman to (Terraphone, peltor, stilo) adapter(s) or a graytronics to (terraphone, peltor, stilo) adapter(s), it's a hell of a lot easier for me to just buy one that's already developed.
KF7RWG
http://www.utahrallygroup.com