The 11C is a 2-piece razor and a bit heavier than the 3-piece 15C. Otherwise geometry of the head is (near) identical. Some like their razors heavy and others don't. The extra weight of the 11C is in the handle so balance is different as well.
It is all a matter of personal preference or YMMV if you will. Usually the extra weight is felt as more aggressive but as far as blade gap etc. goes they are the same.
Yes, the new R41 has the same Merkur head as the Merkur 3-piece OC's (15/25C). Fit and finish are of substantial higher level with the Mühle though.
In fact with the Mühle in my arsenal (and the 15/25C) there is no hair in my beard thinking of ever buying a Merkur open comb again; not that they are not good but for a tiny bit more $$ the Mühle is simply so much better. Looks, perfect blade alignment (never seen that on a Merkur) and handle are close to perfection and make my Merkur look like a factory's 2nd choice.