I use the grip for the M8 by PhotoEqup and find it keeps my hand from cramping when I carry the camera in hand. I don't find it too large but I don't know how it compares the Leica M8 grip.
I also use their earlier model on my M2 and really didn't like handling the camera until I did. My Bessa R2a was more comfortable to carry for me in this way.
I have a Thumbs-Up as well and it helps grip too, but sometimes I dare to use a little flash and the Thumbs-Up does make the hot shoe into a cold shoe. If Leica restored the PC port or the Thumbs Up figured out a pass-thru capability, I'd use it more.
As things are, I carry the M8 with the PhotoEquip grip all the time and slip on the Thumbs Up when I expect to be carrying the M8 in hand w/o flash most of the time, eg. street shooting or candids at parties.