Yes a ZT with the same size 60" deck will mow your lawn a little faster. Also get stuck much easier. Also rotten on rough or steep ground. Mowing yours in 1.5 hrs is quite good really. Unlikely worth a big investment to improve on it.
That is what i am thinking after all a Z400 is $6000 plus.
I think i save some time by not backing up at all so i am constantly going forward mowing grass.
I have a few trees and i can't go under them the rops catches even when down. I think a zero turn would have the same issue.