i tested with nfactor 12 (hashrate like 40KH on 280x...) and it didn't work (block rejected)
tested with a fork of shp-sgminer, param is "--algorithm adaptive-n-scrypt --nfactor 12"
I was having some rejects on x11 and x13 algo at the very beginning. So I went to scrypt N and I had 0 orphans so far.
Yes, nVidia (cudaminer) works fine.
But AMD (sgminer/vertminer) is getting only rejects on N=12, or miner just crashes