Your concerns on the pressure cap blowoff values mirrors my worries when I fitted mine several years ago & like you I tried to ascertain what I was letting myself in for if there was a problem.
It sounds like you have approached the research with more tenacity than I, however the outcome has been the same; neither of us know the answer.
I chose to chance it & up to press it hasn’t been a problem.
My car is a 98 Mk1 & most of the pipe work is original, the replaced bits are associated with the fitting of a PRT.
Firstly I fitted it at the front under the spare wheel well, but I wasn’t happy with that, this involved cutting the radiator top & bottom hoses & mounting the PRT on a cross pipe. This was removed & with fresh pipes on the rad’, moved it to the rear of the car as per later tf’s. When I say fresh I mean second hand, removed from a known car.
The upshot of this is I’m flying by the seat of my pants with a pressure cap rated at 50% up on recommended, the oldest pipes fitted are as built & the most recent pipes only a few years younger. The original mild steel rad’ pipes are now stainless & I have a dual digital thermometer fitted, one sensor on the send & one on return. I also have cooling fan monitoring, override switches & am playing with pressure monitoring.
Most of this trickery is totally unnecessary but I have the skills, the knowledge, the time & a budget.
Many of my ideas have morphed into designs & moved away from their original purposes to find homes elsewhere.
Happy days & I realise you’re little further forward but you have a fellow traveler with me & if I have any problems in the future I’ll post on here, as I suspect you will. Mike.