differance between 13500 BTU/Ac and 15000 BTU/AC

I have a 15K but can't really tell you it would make a huge difference. If it's 100+ even our second 13.5 doesn't make a dramatic difference.
If I were you, I would add either a second, smaller AC that your 30A could live with or get a portable unit and duct it outside. The portable have a lot more competition so they are often much more energy efficient.