The R7 250 2GB from MSI is a fantastic little card for High N factor coins, but the current champ is this little Sapphire R7 240 4GB version.  My theory was low shaders and high memory would be good, and it truly turned out very well.  The GPU mining software needed some additional features to be able to work at higher N Factors, and I was able to find what I needed and implement it into YACMiner thanks to Kalroth (who got the feature as a feature request on his version of CGMiner) : --raw-intensity.  It allows such fine control over how much work is going to a card that I was able to up the output ~ 50% compared to mining software without it.  The reason is that the -I setting sends work to the GPU at a rate of 2^I simultaneous threads.  Well, the optimal rate for that card was 3072 simultaneous threads at Lookup-gap = 4, which fell halfway between I = 11 and I=12.

You can get a decent hash rate out of most any card - just the newer ones tend to just work better.  N=15 and N=16 will be no problem for GPU's either, provided they're high memory cards and you have the system RAM to allocate the whole buffer in one shot (-g 1)