Me neither and I too own both, but then I'm not an ANSTI machine.
I found the following in my old Apeks manual.
ANSTI(1993)
1. TX50......1.05J/L at 70m
2. TX40......1.12J/L at 63m
3. T20........1.12J/L at 63m
All the moving parts of these three regs(2nd stage) are the same and they are interchangeable. And the only differences are the heat exchanger(none on T20) and the adjusting screw. I would be very surprised if they contributed any on the performance of the reg. I would love to see the result for both TX40 and T20 at 70m.