I've just seen the preview for ultimate x-men 46, with the ultimate version of sinister. 🙂
http://www.milehighcomics.com/firstlook/marvel/ultxmen/
He would have been watching the Grey family for years, as he was once married to one of Jean's ancestors. I could see why the offspring of jean would be super powerful, but there is nothing really special about cyclops (except for the fact that his eye beam power is based on some kind of cosmic power source)
Enough.
In X-factor 39 he uses his shield to hold off; Iceman, Dazzler, Storm and Jean Grey... after breaking out of being frozen by Iceman.
Psylocke tries to blast him with some TP, but he resists and fries her. Then rogue flies at him. When Rogue tries to drain him, he switches bodies with her for a little bit.
He's ignored Havok's attacks completely and not been hurt.
In a nutshell... Sinister is like a weaker Apoc.