To get up to 27 MH/s speed with your card you will need to go into Wattman and bump that memory up to at least 2200 Mhz.  For now why not try to slowly bump up to see what your memory can handle the 4GB can't all handle 2250 Mhz.  I would try to bump to 2000 Mhz and see if it is stable.. if it is than bump it up in increments of 50 Mhz until you find where it starts to go unstable.  At that point start to bump down by 5 Mhz or so until you go your max stable speed.  I get 27-28 MH/s eth and 330-338 MH/s with Sia dual mining with stock settings and only memory bumped up to 2200 Mhz.

On my main desktop I run my R9 280x and RX 480 8GB just to get a bit more hash power.  Since I am using my RX 480 for games and such I set intensity -ethi 3 on my rX 480 and I am getting 22 MH/s with no lag in my games I play locally, browsing, or watching video.  So these cards can handle it just fine. So even with my low -ethi setting I am still getting more than you. Get that memory bumped up first and after all that is stable than play around with BIOS mods.