Griot’s specifies on their site that that HD 3” isn’t for use on the G9 (going as far in the Q&A section to mention that it’ll void the warranty). During the YouTube live video, they mention with the 3” backing plate you actually leave part of the counterweight exposed, in addition to having the shroud blocking your field of view. That 3” plate was specifically weighted to work in harmony with the counterweight in the GG6, and I’m sure the weighting is different in the G9 with that ever so slightly longer throw.
I’m sure folks will try it though, so we’ll find out soon enough. Not sure how a 4” plate would work out regarding those factors. I can’t imagine running a 3” (or I guess 4”) pad at speed 6 on this thing!
Got a chance to run my G9 yesterday for some quick testing on a trunk lid - initial impressions are great! Feels really well balanced in action.