engine water temp will not exactly be red hot will it? a cossie water system normally runs around 90 degrees. presumably you would take the coolant as it comes out of the rad so it is at it's coolest? the amount of cooling it will do will depend very much on what boost you run, as this (along with the inlet air temperature) determines the air temperatre after the compresor. the higher the compression ratio of the turbo compressor, the higher the air temp and the water/air chargecooler can be very effective, even using engine coolant. i would also use a separate cooling loop though. i've got a bosch electric water pump sitting on the shelf waiting for this kind of application in the future.

if you do mount an additional radiator, here's a tip. obviously mount it in front of the normal water rad, somewhere near the bottom. don't mount it directly back to back because you will pick up heat from the main rad, but mount it about an inch away and seal all around the edges to the main rad with foam or rubber or something. then in traffic when the rad fans come on,they will also pull air through the chargecooler rad