http://usergroup.plus.net/prodcomp1.php allows to compare products. It shows that there are no restrictions on maximum speeds all day, any activity on both Unlimited and with the Pro add on.
If you look at the legacy products such as Extra you will see that speeds were restricted and then adding Pro would make a difference.
Why they ever made the Pro add on available on Unlimited is a mystery. The only reason I can see is that it increases Plusnet's profits by £5 per month while doing nothing for the user and possibly making the experience worse.
jelv
Plusnet user since November 2001 - not sure for how much longer