Yeah I'm on 13.12 drivers and I refuse to upgrade. You might need to start looking at different TC's to tweak the speed. I've seen a number of 290 configs using TC 32765
I used to have 24550 on my 290's, but the 2% rejects started to bother me regardless of how great the hashrate was. I changed it to 25600 and now my reject % is below .5%.
I think 27000 is the magic number for 290x and nscrypt.
Now, I wish I could get X11 to work right with The Stilt's BIOS... It's great for Scrypt and N-Scrypt, but it really doesn't like X11 and X13 for some reason.