Well I'd like to share a little bit of my own research with trans coolers. The question seems to arise on which is better, to run in series after the radiator or bypass and run a external cooler only.
My first part of the test was running with the factor cooler only as a baseline. I averaged 165deg with a hi of 170deg and low of 160deg over a 5mile drive with both hi-way and city driving. The next part was running both the factor and external cooler, during the same drive I averaged 150deg with a high of 160deg and low of 140deg. Next I bypassed the factory cooler and averaged just about the same temps. On the last test I added a external filter b4 the cooler and again the same temps. Engine temps were averaged at 165deg with outside temps in the mid 50's. Veh runs a 2800 stall

 

Where did you mount the cooler?
I think you should try driving longer than 5 miles.
Does your car typicaly run 165f average all the time? If it does its not good. None of these engines should be running below 190f.

 

Sorry veh is a lt1. Avg engine temps run around the 170ish mark. I have driven the veh on 20mi trip and have seen avg's of 155deg on the trans temp.

 

You didn't mention what kind of stall your running.. I havn't seen over 165 degree temps on my tranny, 3000 stall. And you need drive further than 5 miles to warm up the tranny. Mine would maybe read between 115 and 120 after 5 miles. All depends on how much your converter is unlocked during those 5 miles I guess.

 

Im running 2800 stall. On a straight highway with the coverter locked I was seeing 145deg's on a cool 55deg day. I was really showing under my testing there was little to no difference between running the factory cooler and bypassing it. All test were done with the veh at operating temp. I just used a 5mi local mixed city highway run as a standard for the test. Did'nt want to be doing 10mi runs for each test.