Thank you all for your replies. They were helpful.
I searched for hours yesterday before posting my question, (Lexus dealers were closed Sunday) but I never found any mention of towing capacity being reduced by anything.
I just called the local Lexus dealer and talked to the shop foreman. They connected me to him after I said I needed to know the answer for sure; no guessing. He confirmed that the rated towing capacity is good right up to the max allowable weight for the car, which is GVWR; 5,665 in this case.
Having learned that, I now can be sure nothing will be over stressed or overheated if I tow a vehicle heavier than the 2,000 lb towing capacity as long as the weight of the RX is under its max allowable weight (GVWR) by at least the amount the Miata is over 2,000 lbs (a 1444 lb gap between curb weight and GVWR will allow me to stay well under limits). The weight changes offset each other. The only difference would be the location of the weight (more in the back, less in the front), which must not total more than 7,665 lbs. Ordinarily, I could not assume that moving more of the dog's weight to its tail would be safe. But the fact that the towing package which allows an additional 1,500 lbs behind the RX contains no chassis mods indicates that it is okay.
You asked about hitch specs, how often I will tow and where. A Curt Trailer Hitch Receiver fits the RX. It is a Custom Fit - Class III - 2" Item #C13272 from etrailer.com. It is rated for 600 lbs of dead tongue weight (I won't have any), 1,000 lb with wt distr and 4,000 lbs of towed vehicle weight. I plan to tow the Miata 1,000 miles between TX and NC twice a year. At the eastern end I climb up into the Blue Ridge, but as discussed before, the car is rated to tow 2,000 lbs anywhere at its GVWR. That doesn't mean it's smart, though. I like to baby my cars, so I will certainly install an auxiliary transmission cooler, am considering putting a fan on it and might install an engine oil cooler. I will not put either in front of the radiator. And I might unhook before the steep parts, park one of them, then go back and get the other. Interesting fact: The RX, although it lacks a transmission temp gauge, monitors it and alerts you to pull over if it gets too high.
If you have installed either type of cooler on a V-6 Highlander or RX I would like to pick your brain.
Thanks.
