So for -80C, just r123 and r23? You think r23 will condense at all?
Most definitely it will (at the final stage), and it will go into solution with the R-123 in the early stage, thereby evaporating at the top (suction side) of the cascade condenser somewhere around -50C. Assuming you have a larger differential pressure due to smaller cap tube flows (7-8 foot x 0.031), the discharge pressure running through the cascade combined with the -50C temperature should have no problem condensing the remaining R-23.

If you keep the suction pressure in the evaporator low (5 psi), you should see an ultimate temperature of -80C. Of course for better load handling, slightly shorter cap tubes (higher flow) with a higher suction pressure, would be preferred (although evaporator temperature will be warmer).

Some of this is off the top of my head, so I might be off a little on the cascade temperature achieved, but I know from actual experience with a very similar sized system that -80C was possible, and reliable.