The Marathon Plus, don't even go there.
As I may have mentioned before, I rode the 2010 BWC on a Brommie shod in M+ and finished a respectable 96th (IIRC). Imagine what I'd have achieved if I'd been using "fast" tyres!
Anyway, my experience of the Stelvios is that they're not
significantly faster than M+, so if Kojaks are slower than Stelvios, I really wouldn't bother. Although having said that, it could just be that I'm a faster rider generally than I was when I had the Stelvios, so maybe don't use my experience as a reliable yardstick.
These Primo Comets sound interesting...
d.