I went through this process last year with my Zeon. Did the research, agonized over what to replace my batteries with, looked into NICad, considered modifying the battery compartment to accommodate bigger batteries....
An old-timer gave me some advice which I took. He said the Trojans were great batteries. But, they weren't $100 per battery better! After giving it some thought, I went with a private label Deca (Ellis) battery (around $100 on sale). Its Amp/Hr capacity was the same as the Trojan. At the end of last season I could tell there was some run-time loss, but I had an old golf cart that required a set of 12v. Ended up putting last years set in it and I bought a new set for the Zeon this Spring. So, I didn't get to totally test the theory.
As you know battery life is a function of the manufacturer's quality, how much lead they contain, how well they are maintained, and how much they are abused. In the GE tractors, it not uncommon to get 6-8 years out of Trojan T-105's. But, there is more lead in them to degrade verses the 12v batteries.
The four-bank charger was a wise investment. It will reduce the amount of off gassing.
So, what I have accepted was to shoot for replacing every three years. If the old batteries still had some capacity, they can be sold on Craig's List for more than scrape or core value.
I have parked my Zeon for this year until I can get the stones, rocks, boulders, and stumps out of my new yard. Hopefully, by next Spring I can partially mow with the Zeon.
My two and one half cents worth...