Sector Siege needs improvement

Well, fog of war for one. It plays a big part regarding performance since all human units will update it. Additionally Harbinger is limited to 3 human ships while sector siege can have up to 10, plus a bunch of fighters. Sector Siege has much more going on in a sector than Harbinger, capture points, enemies fighting each other, AI planning moves, Random event generation etc.

The game has been extensively optimized and analyzed to be able to support the unit counts it currently supports.

If you mean the asteroid belt backgrounds from Harbinger the answer is no unfortunately. Harbinger has a background generator that cannot be ported to the other engine without a lot of work. It would have to be coded again essentially. Sector Siege has its own and while it is not as pretty it is very cost efficient regarding performance.