I signed up with Plusnet in May last year, was originally on Value with no pro add-on. I added pro after 1 month, only reason for it was the p2p throttling. I never saw any adverse effect on other protocols, but then I only really used Browsing and VPN.
Since then, my profile has changed a bit to also do a bit more streaming and online gaming (with Onlive (bandwidth hog that it is)).
£5 a month is not to bad for a near line speed connection (please remember that some protocols are still "managed" under pro). If I really wanted to I could drop Pro and just schedule my p2p stuff to only take place from 00:00 to 08:00, when it would not eat my usage allowance, and would download at line speed (or at least it would between 01:00 and 07:00).