i had a customer with a similar problem. an old house that was added on to. the original system was a 5 ton, then another 5 ton was added on. owner had problems with smells, humidity control, etc. these two systems served the entire second floor and a big room on the first floor. as it turns out, the total load was no more than 6 tons and luckily, the add on system is a carrier infinity system and is located at one end of a built out third floor. also, this third floor has chae enclosed on both sides suitable for duct runs. long story shorter, i was able to delete the original plain jane system and utilize the infinity to create 3 zones with dampers and infinity zone control. works great and serves all areas well. since the blower is already variable speed and the compressor in this system 50/50 two stage, it was the ideal arrangement. i don't know if you have the situation to do so, but maybe you can look at a zoning option. as you know, more is not better in our world.