Yes "Happy Pappy", you will be too rich by 14.2% WITH CARBS,,,,however, most modern EFI systems take into account the local air pressure and adjust automatically...
We old school types still have to swap out jets to keep things in the sweet spot.
As to the fact that many vehicles slow down after running several miles, the basic reason is quite simple...Heat Soak...
Assuming that nothing is wrong with a vehicle mechanically, an engine will continue heating for quite a while till thermal equalization is met where the heat output equals the cooling available in all the components...
For every 11 degrees F that the intake fuel/air charge heats up,,, a 1% power loss is experienced....
Robert