Actually an 80amp alternator only has about 35 amps at idle, 700rpms
Engine needs to be at about 1,600-1,800 rpms to get the full 80 amps
So all the vehicle needs is 30amps with all electrics on, fan on high and power windows rolling up or down
Head lights dimming at idle is not "normal", lol, no car maker specs in an alternator that can not power all STOCK electrics at idle.
Dimming head lights mean one of the three Fields in the alternator has failed, so you are not getting the full 40% of alternator capacity at idle, since you don't spend much time idling it is not a big concern............yet, but it does mean you should keep your eyes open for alternator sale
About the 200 amp alternator, one thing I forgot was that the 2.9l used V-belt not a serpentine belt.
When you change to a higher amp alternator it will require more horsepower to turn it.
Outside of the MPG drop there is also the matter of friction so V-belt doesn't slip, think 100hp engine and 6" wide tires, they don't slip easily, now think 250hp and same 6" tires.........you can "smoke 'em" easily, lol, thats friction, or lack of it
I am not familiar with the "wrap" on the 2.9l alternator, how much belt contact on the pulley, but if you start to get squealing, belt slip, you may need to paint the groove in pulley and use anti-slip treatment often