I would be counting myself lucky on your line.
I'm also with PN and have been since 2003, we get these congestion issues every so often, then PN buys some more capacity and the problem goes away for a while.
I do count myself lucky that this line has been remained able to obtain sync at the full rate, but it isn't without local issues, And TBH if i had known how FTTC would perform on this line prior to ordering , i would of chosen a different ISP part of my strategy when choosing isp was what if it performs very badly ,clobbered by dlm ect ect, but the relatively low costs of any remaining months of the contract was affordable, in a worst case scenario where i should have migrate within the min term,
We all have different expectations of how a connection should be, in part because we all use them differently for me a heavily interleaved connection is a big no no, interleave is the enemy low latency and good peering/routing to me is possibly more important than the top speed ,
The ISP's congestion problems shouldn't be happening and when they do they take far too long to react and actually add the capacity required , this secondary IP profile , being set lower than the bt one is saving plusnet a lot of bandwidth, Maybe that what it's really about? I dare say it could also be akin to throttling everyone's connection,?
if you don't have several devices or people sharing your connection then IMO you don't require their prioritisation /shaping , it is of no benefit to me,