I never had a ZD1511, but sold a ZD331 for a replacement ZD1211 last year. I do miss the power, but the 1211 does fine for my use. I find that the vast majority of the time the power is adequate, but there is a narrow window of what will cause it to fall flat on its face.
I had to slow down a few times this season in somewhat thick and wet grass because it was on the way to stalling. The 331 would not have been affected. I also routinely slow down when mowing up steeper hills.
My thoughts? Honestly I did not want the potential hassle of getting involved with a ZD1511, so I went with the 1211. I made a similar decision when tractor shopping 6 years ago with the
B3350, but went up a few sizes to an
L4060.
I mow 30 acres of rural lawns and can't afford a parked Regen or other problems during the course of my day. I hope you get your 1511's problem worked out.