Thanks for the replies. I'd seen the SRX300 didn't actually list the module (but only after I'd tried it) but as you say couldn't actually find anything definitive about support for it. It would have been nice if the firewall would have rejected it instead of recognising and causing a load of issues!
Anyway yes I'm switching to the built in port. We were only looking to continute using the mPIM for historic / cable labelling / paperwork reasons.