Keith
These are loaded questions. Not everybody agrees the additional cooler is absolutely necessary, but having said that there is very strong evidence available that the on-board liquid/air cooler in the rad does not do a sufficient job in all situations.
So IMHO the answer to your first question is Yes, and I will give you chapter and verse on it if you want to know more.
Most people put the slim cooler between the main rad and the aircon cooler. Phil [b3lha] has fitted his one in the front wheel well, because he bought the thicker type that would not fit between the rads. So far as I know it works well for him, but the simpler fit is between the rads. Also there are not too many negatives for putting it there, it actually suits the pipe line run.
The usual model to use is the B&M Supercooler. Mine is the 19,000 lbs GVW model, and will cool the transmission on its own, even bypassing the OEM cooler. So it is not even necessary to retain the OEM one. However, if you are in a cold winter climate place, keep the OEM one in the loop, it warms up the transmission oil to operating temperature quicker.
Feed in series to the OEM, feed the out pipe to the Supercooler, return the out feed from the Supercooler to the transmission.
Just as a point of information, the company in Glasgow that refurbs all automatic boxes in the UK for Subaru UK, they will not give a warranty with a rebuild unless a cooler is also installed. That's how serious they take the need for an additional cooler.
Joe
