As far at the BT Wholesale side of things go the old Office - or Premium as its now called - gets better priority on the BTw part of the network from the Exchange to point of handover to the ISP.
20:1 and 50:1 contention ratios have now gone out of the window and instead replaced with a minimum level and priorities. So say if there was severe congestion at the exchange then its 400 kbps for Home and 800kbps Premium
When I used a BTw based service it was worth it for me to pay the extra for Max Premier as my own exchange historically suffers pretty bad from congestion at peak times. I used to see some pretty low speeds when on 'home' and although I would still see some on premium - it was so much better.
Its worth pointing out that I also saw a shift in hours when congestion was most likely to occur.
When on 'home' it was worst in the early eve.. but when I moved to premium I used to see some busy periods at say about 9am on a monday morning.
The upload was worth it too
Its also worth while checking out with the ISP if the Premium products get better priority on their part of network too. Some ISPs do, some dont. Generally speaking - but not always - it refers to the BTw part of the network..