The 'ac' devices may have more sensitive receivers and do have less powerful transmitters.
Pairing 'ac' with 'ac' should give similar result to pairing 'HP5nD' with 'HP5nD'.

However, pairing a '5ac' device with a 'HP5nD' may have different SNR values at each end.
On the 'HP5nD' end its less sensitive receiver will be hearing a less powerful transmitter.
On the '5ac' end the more sensitive receiver will be hearing a more powerful transmitter.
I expect that the SNR and throughput (TxMbps) difference at each end of a mixed device links will be significant.

It is likely that upgrading from 'nD' to 'ac' devices should be done in link pairs or channel groups.

73, Chuck