The ZG and ZD do not use the same tranny. Infact if you look at the sticker right on top of that you can clearly see that. Same design, yes - same pumps & motors - no.
Its never been my experience that these have lacked for power, and really its been a very good machine. You've gotta realize that Kubota has a very low tollerence for failure with their machines, and we have done work on a higher % of ZD's than we have on the other products. Not major stuff, for us its been mosty the o-rings around the axles.