I was looking to replace my stock speakers too, but I couldn't find any compatible option.
1.-
Bavsound is developing a set of speakers for the F15 since....last year or so?...no news yet...
2.-
Bimmer-tech just launched its first BMW compatible amplifier ($799+free shipping)...but again, a better amp using the same poor speakers, doesn't improve anything...
If anyone find any other option...let me know!