The settings I use on other scrypt coins give me massive HWs too. What I ended up with was 8192 thread concurrency, I 13, and 2 gpu threads. That is working pretty darn well with no HW.
Oddly enough, disabling crossfire, restarting and enabling again seems to have resolved the issue. Everything I mentioned previously works now, no crashing.
I'd like to know if anyone has had any luck at all with the Sapphire 290X Tri-X and what their settings are?