contention ratios

[#1]

I rarely hear these mentioned anymore.
As I am considering switching from PAYG to BBYW opt2
will I notice any difference?

[#2]

Contension still exists but is never mentioned by BT and thus by PN anymore.
Nope as far as contention is concerned but you may notice a speed difference due to different traffic management allowances. PAYG was the least affected protocol whereas BBYW has higher restrictions.
See http://usergroup.plus.net/070425_bbywcomp2.php for a comparison table.
Also remember peak time is 8am to midnight on BBYW, 4pm to midnight on PAYG.

[#3]

Not correct.
On PAYG there is no peak time but you are only charged from 8.00am to midnight