Best i can find is
this but it covers SP60's that are being dished out instead of SP30's. SP60 apparently on average give a greater increase in your premium. I can't find anything to suggest there is even a difference between SP30 or SP50, a LOT of people have them now days. The more popular (want of a better phrase) they are the less they should effect your premium i would imagine.
I think if i had to choose i'd rather have the SP50 i think, i'd rather speed on the motorway than in a built up area if you understand me? They may look at it that way? :Shrugs: