To me on my 5.4L, the difference seems like day and night. When I don't have the A/C on I can floor it pretty good and it'll go without much hesitation. With the A/C on, its like a whole different truck. I'm not even talking turned all the way up or anything, just on the 1st or 2nd level. It bogs so much it feels like the alternator is dying (I've had that experience in other cars) like you can be stopped at a red light, and when you floor it to go, its like you have ot lay on the gas just to get it to go. then once you get going you seem to be back near regular power, like its suckin juice or something.

 

Sure I do feel the loss of power but on the 5.4L it pales in comparison to other engines and vehicles. My 2000 4.6 kinda felt the way you described, it would dog a little with the ac on but it was tolerable. My old 99 4.2 manual was a little dog with the AC on but I suspect the efficiency of the manual tranny made it bearable.

I still chuckle when I think of my old 88 GrandAm with the 2.5L... I'd say about 50% of the power went out the window with the AC on, not to mention that it sounded like a diesel.