I finally got x13 working. My hash rates stats for a pair of r9 280x are: x11 is 4.2x scrypt, and x13 is 3.1x scrypt.
Since WP assumes 4.0x and 3.5x, I actually need x13 to be estimated to be 20% more profitable than x11 to break even.
best way would be to calculate for your setup with help of
http://wafflepool.com/api/stats and then use your ratios
/* per miner */
define("SMHS", 2.600);
define("NSMHS", 1.300);
define("X11MHS", 10.750);
define("X13MHS", 8.725);
define("SPOWERUSD", 8.570/2);
define("XPOWERUSD", 5.050/2);
// i even take power usage into my calculation (mostly to eliminate scrypt-n) and to get actual profit-loss at given time
to make me happy my next step will be closing outgoin ports with iptables so miners mine what is the best choice at the moment.
http://shrani.si/f/2a/xp/1mm84emR/screenshot-from-2014-07-.png