Not much good data for Black Ice Extreme radiators, but BillA HAS tested the thermochills:
http://thermal-management-testing.com/ThermoChill.htm
Just compare the 120.1 and 120.2 and you should be reasonably close to the BIX vs BIX2. As a stab at comparison, cut flow rate in half of the 120.2 and then double heat dissipation for the 2x parallel rads.