Didn't play for like a year here, but din't find any warlock changes also -_- So my exp can still be close to the truth. Any dps class from any spec can beat 15k.
And difference between affli destro demo is not 4k for sure.
There is a pug run with 17, also i had better results with guild runs, but boss dies way too fast there (so every class dps will be increased). Compare to other specs: with affli it was smth like 19k, and 18.5 as demo. So difference is not so high, also my gear was not bis (u can reach more with trading some haste for crit).
_https://www.youtube.com/watch?v=TstXKHXSAOc
So feel free to play whatever u like, but u have ez way of joining groups with higher requirements than u currently have as demo, no idea why not to use it.