If you are running a 15W 40, you might as well run a 15W50 instead (or even a 20W50 full synthetic). For cold weather starts, it's primarily the lower number of the grade that is important. There are also some 10W59 oils available and because of the dry clutch and separate transmission, you do not necessarily have to use a motorcycle specific oil.
What is the lowest temperature that BMW considers OK for use of 20W50? A 20W50 may be suitable for winter where you are.
Another thing to consider is that oils have gotten much better since the 1150 was introduced. One of the reasons BMW recommends a thicker oil is because the oil will shear down and get thinner, A modern 10W40 or 5W40 full synthetic may actually be thicker at operating temperature than a conventional 20W50 after a couple thousand miles after the 20W50 has sheared down.