I never had a Zeiss scope, though I do have two sets of their binos. I used a little known brand called Seeadler (of Nuernberg) when I was in Germany. They used to be awesome
optics, but now are made in Germany, with almost all parts from Asia
(see 'made in China' postings) I think only the turret caps, and reticles are made in Germany now.
A 30mm tube, and a 56mm objective was a German standard, but now that outside scopes (Leupold, Burris) are creeping in they are going toward US sizes.
The theory is a wider tube, transmits more light. NOT TRUE! But you do get more adjustments. And the 50mm or 56mm lens really does not mater. In Germany I used 40 & 50mm scopes, AT NIGHT, and I couldn't distinguish much between light transmission between my 40mm, 50mm and 56mm scopes. It's is all lens quality, glass, and coatings that make the difference.
I do not think it makes a difference, because the Zeiss has such superior optics, your getting a super scope anyway.
I would send John Barsness a PM or email, and ask him, I am sure he would agree with what I said.
After the US Government gives me my promised promotion in September I am getting the 3-9x Conquest. I am sure it's quality could handle any low light problems I may encounter.