I own and operate a J400, 4 seater Jabiru... PM me if you want any specific (correct and precise) information. There is much hear-say on this site which is usually mostly incorrect when it comes to Jabiru's.
The figure of 120 knots on 20lph is pretty much accurate.
As for Jabs being a "pig" in turbulence, this isn't exactly the best description.
PM me and I'll fill you in or give you a call.
I also have a mate with a Tail-Dragger jab so I can put you in touch with him if you want. Jabiru don't make tail-draggers any more, however there is a gentleman in the process of building a J200 tail-dragger convertible with Rotary 200hp engine! stay tuned for more on that one!
Intertia in the bigger jabs is definitely increased over the little ones. The J200/J230/J400/J430 have a massive amount of inertia giving smooth zoom climbs of around 800ft with ease.
The Jab will produce a far better pilot than a Gazelle ever will. Converting to GA (C172 / Archer) is a complete non-event for a Jab Pilot; the problem usually lies in the reciprocal with "normal" GA pilots unable to successfully complete a flare in the jab for the first few landings.
As per the Jab website, a J400 *WILL* at MTOW out-perform an EMPTY 172 !!!