Does that mean that line quality on the all in one Plusnet packages will be the same as BT?
The line quality (ie speeds/snr/attenuation) will be the same across all non LLU lines, regardless of ISP, as it terminates on the same BT Wholesale equipment, and uses the same Openreach lines.
Potential throughput will vary, as long as its not your line or exchange congestion causing the bottleneck, as different ISPs load their Centrals differently, and have different connections to internet once you're on ISP's network.
Plusnet and BT's branded ISPs use different infrastructure, and are run as seperate entities, so expect different results between them. Plusnet in particular are swines for traffic shaping, and the idiots traffic shape VoIP (which in effect blocks it) unless you use their own VoIP service. Naughty.