One thing to consider when you're replacing parts (like a arms) is that too stiff just causes other issues. You need some flex in there, and going to a setup that is very rigid is just going to break other things; and they will cost more than a arms
It may sound backwards, but its possible that even the stock stuff is too rigid for the beating you're giving it
Has it been cold where you've been running?
Sometimes, its not about making the car stronger, its about keeping it off the pipe - at least thats what I keep telling myself with my ZX-5 SP