apocalyps is going to use this one:
http://www.deltaww.com/products/dcfa...B120120762.pdf
Otherwise, you're hitting a barrier with that rad, and I don't believe that it'd be worth to switch to a BIX (maybe another degree or two?). If you sell your BI to get a BIX, you'll be out the same amount of money you'd fork out for a new heatercore.
If you just buy a heatercore and sell your BI, you might recoup most of the cost, and you can then drop your temp maybe up to 5 deg C, it depends on the size of the heatercore.