Carbon Leather with a black interior will be a lot of black; for that reason I got aluminum (pardon me, "aluminium" where you are
) and I'm very happy with the contrast, but I know some don't like it or feel it's been overdone in cars in general by now.
Otherwise, I'm personally biased towards a manual transmission (my current M3 was the first manual car I'd ever driven and I'm hooked), but I'm sure you've already read at least some of the numerous threads on the subject, and I can't deny that DCT is an extraordinary piece of engineering.
I think heated seats rather than CWP is absolutely the right move (though I didn't get either one since my last car had heated seats and I never used them), and I have never once wished I had High Beam Assist. In fact if I had it on I'd probably worry that it might not turn off correctly when a car was approaching, at which point I'd be focusing on my high beams just as much as if I hadn't gotten the option in the first place -- though I admit I haven't done research into how well owners claim it works.
I'm not sure about being able to add Sirius later purely by purchasing a license. There is definitely a retrofit option, but it may involve hardware. Hopefully others more certain than I will chime in there.
The only thing I would consider adding would be the Enhanced Premium Sound option. It won't completely blow your mind, but it is definitely very good, and I believe made necessary by the fact that the base stereo is truly abysmal. Then again, I admit that I only listen to it on longer highway trips, because everywhere else I prefer to listen to the stereo located directly under the hood.
Enjoy your M3!