I have no probs with the knucks flying a bug smasher on the weekend in the gloop - they wouldn't have a problem and they're not keen on killing themselves.
The problem would be unfamiliarity with the poor to non-existent single engine climb performance on a Duchess, say, and being suddenly thrown into the twilight zone that exists when you're not necessarily going down, but you can't go up fast enough to make a missed approach gradient, let alone a SID.
I'm not saying I've done heaps of that stuff, but a Caribou posting was enough to open my eyes to the 'joys' of bumbling around below LSALT with bugger-all climb performance.
As for the IRT - that should be cut and dry - got a military IRT, then that should be all you need to fly in the fluffy stuff - make it easy and simply use the same date as your mil IRT, and provided you meet ALL the civil currency and other requirements then you should be good to go.
You still need the IREX to ensure familiarity with the appropriate regs, eg differences between what is done under SI's / military AIS pubs as opposed to working under the AIP / CAO / CAR / CASR etc.