I owned both a Z720 like PSC demo'd, and also a Z950. IMO, neither was or is underpowered either by the engine or from the hydro's. I still don't know what PSC is talking about when he says that. Neither of mine bog(ged) down from pushing the sticks forward or anything else for that matter. You can't beat the QOC that the 7 Iron delivers either.
The JD's do need a suspension seat on them though, no doubt about that. The hydro's are very responsive, especially on the 900's, but they ride like a tank. It's about time that they finally added a suspension seat to their Z's. Too bad you have to buy the top of the line machine to get one though.
I really like Gravely as a company, and their machines are excellent IMO. The next time I buy a Z, it will probably be a Gravely due to the way they, as a company, treat people, and also because the 400 series are excellent machines.