Not the exact same skis, but I managed to demo the RTM80 and the new MX83 a couple of months ago in mixed conditions (4" of new snow over a groomed base, turning to cut up pow and soft bumps in the afternoon).
I'll preface this by saying that I'm a fan of the Kastle MX line, and own an MX78. I have an older pair of Volkl Supersport Allstars, which I enjoy on hard snow. I've never been a fan of the AC50, although I really like the Kendo. So I guess I'm all over the map.
This year I was pleasantly surprised with the RTM80 and would happily own that ski. Grippy and even a bit playful, which is not really how I would describe any Volkl I've previously been on. I put that down to the slight, continuous rocker. Still, the MX83 seemed more precise to me - not necessarily better, but different. Hard to describe, but here goes. Once you enter the MX into a turn it is engaged until you make a definite decision to disengage from the turn. The RTM80 entered a turn easily, but seemed willing to disengage from the turn just as easily.
I'm 6'4" / 210lbs and was demoing the 183cm MX83 and the 176cm RTM80, so take that into consideration. Normally a rockered ski would feel shorter than the stated length, but the reverse camber of the RTM is so slight I didn't get that feeling. They seem to ski to their length.
They're both very good skis, and you could hardly go wrong with either pair. If I had to choose one ski for the rest of my days right now it would be the MX88. If someone 'forced' me to take the RTM84 under the same scenario I'd still be happy.