Peter
On paper, Jan-March is the bad weather season (SW monsoon). I've been to Sipadan 6 times, once during Feb. Other times have been in the "prime" Jul/Aug when viz is suppose to be brilliant. Well 2 times, viz in Aug was terrible, worse than Feb. In fact, we didn't see any big schools of jacks, maybe 50-100 vs the 000s and the cudas were spread out wide.
What I'm saying is the winds are up in Jan-Mar, no doubt. Viz is less than the high season but not the worst I'd seen there (that went to Aug/Sep 2001). One storm can make the viz go crap and that's regardless of time of year.
More importantly, I went in the days where you could stay on Sipadan. If I had to take the 30minute boat ride daily to Sipadan in that rough sea (relatively) and get splashed silly before getting into the water. I'd rather just do local dives in Mabul.
If you are going though, please keep us informed about the new restaurant being built there. Seems Tuan Musa is determined to build his clubhouse there.