bmw v12's arent very good. the first one (used in the 7 and 8 series) was just 2 m20 motors stacked, literally.
used two motronic ecus, one for each cylinder bank, etc etc.. ran dual everything. and had a tendency of failing frequently in one way or another.
you'll get better power to weight ratios by supercharging/turboing the m20 or m52 or whatever you are running right now then switching to a v12..unless you switch to a modern v12.. but if you could afford that swap you wouldnt be worried about it, you'd just find a way to do it (money has a wonderful way of making the impossible...possible)