Just to add to the above, I compared ZX 16 to a soft copoly that I've used many times with gut mains, Proline II 1.25. I looked at the deflection numbers at TW University, and it appears that ZX strung at 60 pounds deflects the same distance as Proline II strung at 50 pounds. And, at 60 pounds ZX exerts a peak force approximately equal to Proline II strung at 30 pounds (so it should be more comfortable even at that high tension). In addition, the energy return of ZX at 60 is 91.6% versus 84.5% for Proline II at 50.
So, if we put stock in the numbers, ZX as a cross with gut mains at 60 should perform similarly to Proline II as a cross at 50, except that comfort and "pop" should be significantly improved.
But one final thing to consider is how gut mains at 60 behave versus gut mains at 50. The tighter gut mains will obviously deflect a shorter distance, but probably with more energy, than looser ones would.
Here's a collection of the deflection numbers for ZX 16, Proline II 1.25 and Toughgut 16 at three reference tensions (these are post-tension-loss data):
Toughgut 30.8 mm
Proline II 30.0 mm
ZX 16 32.1 mm
Toughgut 33.0 mm
Proline II 32.0 mm
ZX 16 35.2 mm
Toughgut 16 36.0 mm
Proline II 35.1 mm
ZX 16 40.0 mm
So, if I used gut/PL II at 51/51 my gut mains would deflect 33.0 mm and my PLII crosses would deflect 32.0 mm.
If I switched to gut/ZX 16 at 62 pounds, my gut mains would deflect 30.8 mm and my crosses would deflect 32.1 mm.
Extrapolating from these data, with gut/ZX strung at 55, the gut mains might deflect 31.4 mm and the ZX crosses 33.7 mm.
I'm not sure how useful these numbers are, but they are something. The wildcard is the different interstring friction characteristics of the ZX versus Proline II. TWU's testing showed Proline II 1.25 to have an interstring coefficient of friction of 0.101, while ZX 16 tested at 0.088. According to these numbers, ZX is actually more slippery than Proline II, but playtest reports suggest that this slickness might not last as long as it does with copolys. This might be due to a coating that gets rubbed off, or degradation of ZX's relatively softer surface.
Just looking at all this, I'm thinking 57/57 might be interesting to try for myself.
Pity that TWU hasn't tested Mosquito Bite.