First thing first, add the following to the start of your batch file:
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
Those would be for windows, I use linux; and I already have the same commands albeit correct for linux in all of my startup scripts.
Now, you need to understand that the number of threads you should be using is dictated by your card (not all cards can run well with 1 thread same as not all cards can run well with 2 threads) so find out which is best for your card (by finding out I mean try it out for yourself).
These rigs are rigs that I have countless hours of tweaking on normal scrypt and have determined that 2 GPU threads is by far the best. I gain about 30% going from 1 to 2.
Next, the number of TC (thread-concurrency) in your settings is capped by how many threads you're using and by how much RAM you got in your system (to determine how much TC you need to set: Shaders*n+1 (this formula will give you, generally speaking, the best resaults), n can and will be any number you choose but is subject to the limits of your cards/settings, although YMMV).
Again, countless hours of tweaking on these rigs and I have identified my optimal TC, but this is adaptive n-scrypt mining so the normal rules don't necessarily apply.
So, for your OP, if with 1 thread you see low KH/s than you should change your TC.
If with 2 threads you still get error -5 (after adding the setx commands) than you should lower your TC. A common TC value that works well with -g 2 is 8193.
I tried 8193 on a whim, and same thing. I really appreciate all of your help, but I think you're perhaps barking up the wrong tree. My thoughts are that this may be an adaptive n-scrypt specific issue, or perhaps a bug in vertminer.