I have coached two men who have run 2:44 and 2:50 respectively. The male that ran 2:44 was actually quite big, 6’2 and 180, while the 2:50 marathoner was 6’ and 150. Their training was quite similar, but they had very different natural abilities and builds. While I do think that weight is important (it is basic physics), it’s only one piece to the puzzle for running a fast marathon.
The pillars of my training philosophy for marathons, are lots of runs at marathon pace, threshold type workouts, progression runs, and VERY EASY RECOVERY RUNS. Most people run their recovery runs too fast, they let ego get in the way. This impedes recovery, and takes away from the real workouts, and often leads to injury…anyways, that’s just my 2 cents.
I definitely think you have the ability to run sub 2:50, but probably need to include more longer runs at marathon pace, and losing a few pounds wouldn’t hurt you either, as long as you are still able to well fuel yourself for workouts. Dieting to the point of having bad workouts is never recommended.
FYI: here is a good calculator I use to show the impact of body weight on performance: http://www.runningforfitness.org/calc/diet/weighteffect?metres=42195&hr=2&min=50&sec=10&weight=170&weightunits=lbs&Submit=Calculate