I think it could be cool to have the AX8 in a Helix send/return loop for yet more amp models available via snapshot/midi in a single Helix preset. I'm pretty sure it has amp models that Helix doesn't so it could be a nice compliment.
Also, I think AX8 utilizes ultra-res IR's up to 8000 samples, so the same IR's/cabs may sound much better in AX8 with either Fractal amps or Helix amps. Just using the AX-8 Ultra-res cabs may take the Helix amps to the next level. Plus, this may give the ability to use convolution reverb IR's and better sounding acoustic model IR's. Again, I'm guessing you would use a Helix send/return block after a Helix amp model. Maybe multiple S/R blocks.
Fractal Cab Lab and IR mixer would be handy.
AX-8 also has a 4 minute mono and 2 minute stereo looper and good control layout.
I wonder if there is a way to set the AX8 up to control Helix snapshots via MIDI full time and leave Helix in stomp and/or preset mode. AX-8 has 8 preset buttons and MIDI capability.
I also love the fact that you can get both of these units for about the cost of a full on Axe FX II and foot controller...sans rack!
Personally, I would probably never get through a whole song if I had both of these units and all of those options in front of me.
I look forward to Miguel's findings...and hopefully some recordings of Helix amps into Fractal AX-8 Ultra-res cabs :)
Okay, one other thing I just noticed is that if you set your expression pedal before changing it from Decay (the first parameter) to Mix, the expression pedal assignment will stick to Decay also. Did you try to go back into Menu>Controller Assign and set Decay to None again?