IIRC last year Oakhill were the "undefeated" premiers of the CAS competition. In a variety of games they played and beat all the CAS teams at one stage or other.
Oakhill were a very good team last year and were heavy with Y12 boys who have now left. By all accounts Oaks are rebuilding this year and will not be as strong as the 2012 vintage.
Augies did not play all the CAS teams last year, but again those that they did were rather well beaten.
Of the city teams Pats was probably next off the rank in 2012. They were beaten by Trinity, so would be competitive without being world beaters in CAS.
Stannies and Kinross would be around the same level.
On their day Andrews, Gregs and Pius would win a few games in CAS, but not dominate, based on form over the last few years.
Despite promising much, the ISA rep team has had inconsistent results against CAS rep team in the Opens.
Bottom line, in a gross generalisation, the top ISA schools are at the same level as the top CAS teams. The 2nd tier ISA would be reasonably competitive in CAS. with the weaker schools struggling somewhat.