I don't normally talk on the HT with the antenna by my head -- when it's in the car, I use the headset and stand the radio in the center console cup holder. As you note, however, that does nothing for the height and attenuation issues. Everything I've seen says the Nagoya 771 I have on there (yes, a real one, not a counterfeit -- I checked) is a huge improvement over the stock rubber duck -- but again, it's still inside the car, and more like two feet above ground than five.
Regardless, I did in fact manage a brief 2m simplex QSO (with my installed mobile, not the HT) on the way home from work on Thursday, my first ever -- after calling a long CQ every couple minutes for above twenty minutes, someone answered back, and by the time he could say my signal was a little scratchy and I responded after turning my power to maximum, I'd gone out of range or been terrain blocked, and we lost each other. The next afternoon, I picked up a local net on 2m, from a repeater that was about 25 miles away at the start and closer to 35 by the time they closed, and never lost signal even down in the low spots. I'm still of the opinion that even with power and better antenna, simplex isn't much use from a moving vehicle in hilly areas. It would almost certainly improve on longer wavelengths, I'd think; 6m and 10m diffract over hilltops a lot better than 2m or 70cm.
I don't see myself trying to put even a 20m capable antenna on my Fiesta -- the quad-band (10m to 70cm) whip with coils I have on there now looks like a CB antenna from the 1970s (though the "better" antennas then, on 11m, were a good bit longer than this one). People must use loading coils and capacitance hats to get antennas on cars that work with 20m or longer. There is NO WAY a 38 foot 5/8 wave whip is going to stay on when I go under an overpass at 55-65 mph.