Any question answered...

If you can notice the 5mm difference, you won't after an hour of riding.

As I understand it the Alfine one is basically the same as a 105.

Nice, Thanks.

It will be, as long as you get your BB shell faced if you haven't done so already. £15 or so at your LBS.

Had that in mind already, will do.
Would it be advisable to spray the now "naked" faced surface with some kind of anti corrosion spray? Since my BB gets wet all the time, I'd expect some rust there otherwise...