What are your requirements for slim AVRs?I actually quit experimenting with matrixing because of my preference for full range overheads, I just wasn't happy with any of the slim solutions that I tried, and would have to completely move my equipment storage to use the proper AVR's to be happy.
For me the Marantz slimlines are just fine and are just over 4" tall each, 8 1/16" tall stacked.
Going over 50w/ch with height channels doesn't seem that likely in a home theater setup. Having L/C/R all driving a constant 50w/ch in a scAtmos configuration just doesn't seem likely. The NR1403's can probably peak at ~65w with 1ch driven. While that is well below the 125W+ of a big AVR even then it is less than 3db of difference.
With efficient speakers (mine are 89db) the height speakers would be way too loud before I ran out of power. The FH/RH speakers are at 12ft away and this should allow them to hit ~96db. TM are closer so they should reach ~103db each. My main speakers at 11ft can really only hit ~100db each even with a 150W external amp. It might not quite hit THX headroom requirements at 105db but it is still louder than comfortable...
If you need an external amp I have an 16 channel amplifier sitting here that can do 5x200w bridged plus 6x80w on the remaining channels... Of course it weighs 95lbs...