Edmond,
With the larger sensors, MF lenses behave in terms of DOF as if they were 1 f-stop larger. That is, the DOF at f2.8 on MF is like an f2.0 on 135 format. Also, since the lenses are longer, the DOF shallower, focusing anything more open than f2.8 becomes more difficult.
The Mamiya 150/3.5 is very sharp. I've taken handheld head and shoulder portraits at 1/30 f3.5, with a Kodak DCS 645M (16MB) back, a will show facial pores when enlarged, if that's what you're after. At f3.5 eyeglass and eyes are in focus and ears are very blurry with background wonderfully fuzzy.
However, if you want to duplicate your 80/1.4, you can buy a used f-series Hassy 110/2.0 with a Fotodiox adapter (run about $80 on eBay). This will give you an 80/2.0 equivalent on a Leaf75/P45 with an effective DOF about like an 80/1.4. This adapter forces you to use the lens in stopped down (preset) fashion, but if you're shooting wide open, its a moot point.
I recently compared the Zeiss CFE 120/4 makro on an adapter on the AFDii against the Mamiya AF 120/4 macro. All comparisons were on a tripod, using an older Kodak DCS back at ISO 100, all images processed the same in Raw Developer.
The Mamiya was slightly sharper and had more contrast than the Zeiss at the closest focusing point of the Zeiss (1:4.5 ratio). However, the background blur of the Zeiss was as if it was shot f-stop more open. I added an extension tube to the Zeiss and re-shot both at a closer distance; Mamiya was sharper, but not by much. At infinity focus, the Mamiya was decidedly superior.
You mention Contax. AF is slower and the viewfinder is dimmer than the AFDii (I compared this with Mamiya before selling my H2). Besides, if get the Zeiss 350mm (the 210 is supposedly a dog; so you go from 140 to 350), this lens weighs as much as the AFDii body and the Mamiya 300 APO combined. You talked ealier about traveling, so weight becomes an issue.
Here are some data regarding mass for all three systems (focal length on left, weight in grams on right):
Mamiya 645AFDii
body/finder/back - 1430
35 - 480
45 - 480
55 - 430
80 - 300
120 - 845
150 - 540
210 - 720
300 - 1430 (1240 without tripod collar)
55-110 - 870
105-210 - 990
Contax 645
body/finder/back - 1530
35 - 880
45 - 825
80 - 530
120 - 800
140 - 680
210 - 1220
350 - 3610
H2
body/finder/back - 800+330+475=1605
35 - 975
55-110 - 1650
80 - 475
120 - 1410
150 - 970
210 - 1320
300 - 2120
As for shift lenses, the there is nothing as wide as a 24mm for MF unless you go with an Alpa/Horseman. The 35XL with the Leaf/P45 is about like a 25mm, but you have no tilt like the Canon 24, but a much better lens (at a hefty price).
Regards, Robert