is check in allowed? can all non-breakable item go into check in? If check in is not allowed, 7kg total is going to be tough. A minimalistic BC is probably 3kg already, + fins, suit, misc safety item, it will go over 7kg easily. In this case saving 0.4kg probably doesn't help either.
Last time I visited Thailand, I stuffed Al Bp/w with STA, 30lb wing, single BM reg set, DUI30/30 drysuit + usual misc item all into a 22" carry on. It is probably over 7kg, but not unreasonably heavy. Fins, clothing and toiletry item went into check in (well under 50lb/23kg of limit at that time). Knowing I have a piece of check in, weight saving was never in my thought. I can see if I am going to cold place for diving, I will need to consider weight saving.
Just curious, the 5.5kg is for BM double for single? I just measure my brass back mount single, standard Hog/DIR kit with drysuit hose, it is 6lb (2.7kg) exactly. 1st stage is Mk25, 2nd stages are G250v. Adding another Mk25 (1.5lb) make BM double. Total of 7.5lb => less than 3.5kg. so what did you in your BM kit?