Of course, the ideal is to have identical subs in a multisub system, but the room acoustics of low frequencies mangles the response so badly that some phase mismatch probably isn't going to be a big deal in comparison. One interesting test would be to have a PC13-Ultra and PC-4000 side by side in a groundplane setting, simultaneously playing the same measurement sweep to see the effect on the response.
We have an international customer (who has REW and UMIK-1) with quad subs in his system - dual PC13U and dual PC4K. We advised him to place a sub in each corner of the room for exactly the reasons you stated above - room modes do indeed tend to mask phase-related cancellation between the two models - and he was able to achieve a decent-looking FR at the MLP.
His electronics provider (who shall remain unnamed) advised him to place the four subs into the front corners (PC13U/PC4K and PC13U/PC4K) for 'better time alignment with the front channels'. I predicted this would be disastrous for a number of reasons - including phase cancellation between the two models.
He ran some REW sweeps of just a pair of co-located PC13U/PC4K and sure enough there was significant phase cancellation over certain bandwidths. If he adjusted the phase control on one of the subwoofers, this merely shifted the cancellations to different frequency bands. Suffice it to say - he immediately went back to placing the subs in the four corners of the room.