The e12/e24 vented front calipers are the same as Tii and all other non-vented BMW 6cyl cars from the same era BUT with a spacers in between the halves to allow clearance for the wider vented rotors (doh!). These are the same front calipers as on the vented rotor applications of the e9 Coupes and 2002 Turbo and other vented applications of e3 6cyl sedans etc...
Yes the Tii (non-vented) and Turbo (vented) had the same front braking power as the much larger 6cyl Coupes and Sedans abeit with 4x100 hubs instead of 5x120, even the same bearings were used!
The calipers you do not want are found on the e23 7-series which are a single line version of the e12/e24 calipers, which are also found on the e28 M5. Yesss Virginia you will have the same brake pad size as a genuine M-car! Just not quite the same diameter rotors.
Due to the tight fit of the '77 e21 320i and e21 323i vented rotors within the e12 calipers (they're not quite centered); I recommend using the Tii pads as they have slightly less material thickness and tend to simply fit better w/o dragging on the rotor! Some cars line up better than others though. I have found no east way to remedy this, if someone else has a solution please speak up.