2008 3.5 power output, is this normal?

[JSHRAM]

Hi there.  We were just given, by my wife's parents, a 123k mile 2008 3.5 AWD highlander.  I've not owned a Toyota before but am aware of how good a car and motor they are.  I'm a pretty seasoned back-yard mechanic and have owned many-many vehicles.  I'm asking because it seems to me that  this 270hp motor, though well maintained and running smoothly with no codes, isn't pulling with  the power it should.  We live at 6000ft, so that affects it, but on the highway, pulling good grades at 80mph this thing will shift down to 3rd, regularly, with no weight in it other than the wife and I.  I could easily just leave  it in 4th, but it would still shift down to 3rd (5000 rpm).   I've never had a car need to shift down so much before on these hills.   In fact, I can't remember any car shifting down to pull these grades, especially with no load.  Now, I  do feel that 5th seems to be an Overdrive gear and is quite high... at 80 I'm turning 2500 rpm, that's low..  so 3500rpm in 4th would not be a shock, but needing  3rd, regularly, and turning 5000 rpm?   Why can't this motor pull?    Is this normal?  Again, no codes, no rough running, stock car, well maintained... plugs are due, I'm about to do  them,  airfilter is OK, transmission fluid is good and  running smooth.    Thanks.  John