I don't know how retrans works in practice, eg. is it out of band from advertised sync,
Retransmission happens using the advertised sync, not out-of-band, which obviously can result in a loss of effective bandwidth.
With retransmission activated, the modems report a "minEFTR" value, which is the minimum effective throughput rate seen recently. I guess if you run QoS too closely, you might need to adjust dynamically based on this value.