Hi Danny,
A major consideration for use of Rebreather in Overhead environments is partial flood recovery / resistance to flooding. While it is technically possible with most units to get rid of water from the counterlung, it might pay to talk to people who have the experience to tell you just how easy it is on the units you are condsidering. I believe on the inspo it is harder than on the meg...but I'm sure there are folks around here that can comment with the authority of having used both units.
As you are probably aware Craig and Karl took Prisms to the back of Cocklebiddy with the two Pauls and Dave Apps on Inspos (see
www.trimixdivers.com for the reposrts). Craig is still heavily into his cave diving and now dives a Meg. He's based in Perth - I'd ask him why he dives what he does. Get in touch through Image dive.
Cheers