I really don't think it has to be that drastic though. Easiest just to have full fleet details for fleets in the players system, and then all other systems have a 'fleet token' that just stores the hull variants in the fleet and the commander stats. AI fleets will have pre-defined variants (no customs) so you don't need to really store much.
Then when player moves into a different system, all those fleet tokens just get converted into real fleets, and the previous systems fleets get converted into fleet tokens.
And there is no reason the frequency of updates in systems the player isn't in can't be lowered to like 1/60th rather than 1/10th (or even once per StarSector day). Anyway, it's all fairly non-trivial I imagine so not something you want to embark on without a real need (and currently there is no real need). If you start optimizing too early it usually makes refactoring/changes excruciatingly difficult later.
re. SPAZ - although I thought SPAZ was a great game, it felt a little too much like a sequence of arenas in my mind. There was too much 'disconnect' between leaving a system and coming back.
re. Exerelin - I wouldn't really view Exerelin performance as indicative of StarSector performance, as it is an unoptimized mod on top of an unoptimized StarSector