what does a 3rd gen rx normally rev at when warm, mine sits about 1300-1400rpms, I am used to a piston engine sitting about 850rpms when warm.
should sit between 750 and 900. depending on mods you have, this could be caused by several things, but probably some sort of vacume leak. Or the cold start mechanism not fully retracting when the car is warm. It reads water temps and a little plunger on the back of the throttle body holds the idle open a little. It is quite common for it to get stuck. There is an adjuster screw which you can adjust to sort it. Only mentioning this possible cause as this is what it was on mine. I now have no cold start, (disconnected) and the car is no different!
I have no idea on the full list of mods but know the ones I can see, I thought it was a bit high reving and only wondered now after a few months of owning. SOmething to check though, thanks m8! :)