I am wondering ...if I do not have the trailer package from the factory and I add a transmission cooler / reciever hitch how much should I be able to tow without concern? Would it be 4500 or just 2000 pounds?
Well, there is another piece you would be missing which is the tow button. It will adjust upwards the shift points to prevent upshifting at the regular rpm, and also keep you out of 6th. You can of course overcome that with manual shifting.
I had a hitch installed and towed a 1998 mustang convertible with some added wieght on a Uhual transport from Fredericksburg, VA to Illinois. You have to use manual shifting and keep an eye on your temps. She pulled it like a champ. Although some hills on the turnpike were sluggish.
Without the TCM control (towing program for the ECU) or manually overseeing the gears yourself, you probably risk a bit of accelerated transmission wear and you would still be missing the larger coolant radiator for the engine. Certainly the trans cooler will take care of some of the extra heat load but my experience has already been that the radiators are not huge in capacity on these. I found while driving some mountain grades in 103F heat a few weeks ago, that the coolant temp rose notably above the halfway point (where it otherwise always seems to stay) without a trailer attached, and I do have the trailer package so it sure made me wonder how much spare capacity was really there.