Can only think it must be an oil leak, or for it to be 'using' this amount of oil there should be smoke I would think - have someone follow behind you and drive up and down some hills etc.
I don't agree with the performance drop when cold, although I always nurse a cold car until warm, but I gather you are talking about a significant drop of power and then having to restart the engine...
These engines are known to use oil, but to what extent is the question. When I first got my 330i it was on the 50% mark and by the time it was asking for a litre top up it had done 6500 km from the previous service. The next time it asked for a litre was 8100 km later and then soon after the car went in for its last service. Have done over 5000 km and it was still sitting on the max level, then at around 5500 km it went down to 75%, so not too bad at the moment.
I think valve cover gasket and heat exchanger gaskets are usually the cause for oil leaks, so have that checked out too...