15% isn't that bad, so long as your reject rate is increasing and dipping along with the pool's rate.  Having said that, I've found the following settings for my 290's to reduce my rates lower than the pool's average and still follow suit too.

For my Sapphire 290's:

-g 1
-w 256
--lookup-gap 2
--xintensity 300
--thread-concurrency 20481
--gpu-engine 1015
--gpu-memclock 1500
--gpu-powertune 22
--auto-fan
--temp-target 80
--gpu-vddc 1.025

For my Asus 290's:

Same as above except xintensity is 124 and thread concurrency is 20996.

Another thing I've noticed with the new sgminer.  If I start the program, allow it to run for a few seconds, close it, then immediately run it again, it evens out the hash and wu quicker as well as keeping the rejects lower.  Not sure why, but it works for me.

Hope all of this helped someone!