I agree with everything said. I've had many cars before, including 335xi and M3. Those cars, without doubt, stipulate premium fuels because of their high compression ratios and turbo. Having said that, Lexus 3.5L engine is essentially the same as Highlander. It's marketing gimmick to make people believe they've purchased a Lexus and it "deserves" to be fed premium fuel. I consider myself a car aficionado. I will never cheap out on gas to risk damaging the engine. But I firmly believe the Rx can and will take regular fuel with little to no decline in performance.
I've read an older post, which is now locked, regarding the prospect of using regular fuel. The person who broached this topic got flamed. Although a few bucks saved for every fill up seems inconsequential, this amount sure beats the interest you garner from the bank nowadays. Food for thought :)
You've provided some very good information here (i.e., that the Highlander engine is a doppelganger for the RX engine). You'll hear from some that you are doing horrible things to your engine running 87 octane; why would you risk expensive damage just to save 20 cents a gallon?; etc. etc. yada yada yada.
Your logic is correct; virtually the same powerplant tuned to make slightly more power output if you run higher octane fuel. Who wants to spend thousands more for a Lexus and have it reported that it is only as powerful as it's Toyota brethren?
I've run 87 octane in three RXs now for well more than 100k miles (some miles at wide open throttle right up to redline) and never heard a ping, a knock, a bang, etc.; every owners manual recommended 91 octane or better (but how much better? - I can get my hands on 112 - should I run this instead?) And I've always been pretty impressed at the power output - never a sign of compromise from my vantage point. The 2010 engine is very impressive.
My opinion: high octane and 5 horsepower is just marketing BS. The owner's manual does not "prohibit" you from running 87 octane; only recommends high octane to obtain best performance.