In that case, I would second earlier comments that probably your biggest improvement would come from swapping that unit out for an automotive heater core design. It will do far more for you than beefing up the fans, and it WON'T make your system noisier.
This isn't just my opinion. If you go to the much referenced BillA rad testing articles, you will note how the heater core style rads clearly out perform nearly every other design on a size per size comparison.
Oil coolers are particularly poor by comparison as they use thicker tubing (slower to transfer heat from the fluid to the fins) and fewer passes of tubing for their area than a heater core. This works for an oil cooler that has to withstand far higher pressure, and has very hot oil going through it, but the design is lousy as a WC rad.
OTOH, a heater core has many more passes, each made from paper thin tubing, with lots of closely spaced fins. It will do a FAR better job of transferring heat from the coolant to the air.
Gooserider