OP said he wants to go faster, not tour. Faster means lighter, more narrow tires.
Buying a road bike that weighs anywhere near the weight of his mountain bike would be a step in the wrong direction if speed is his goal, imo.
To reiterate, a Specialized Secteur with 25/28 tires was fine for me at 325 lbs. Almost 2,000 miles on it now. And much faster than my hybrid which was much heavier and had wider tires on it.
My 2013 Secteur morphed into this bike for 2015:
Specialized Bicycle Components
The 2015 Secteurs look to be good bikes as well:
Specialized Bicycle Components
If you really want steel, could look at the AWOL:
Specialized Bicycle Components
But I've ridden that bike and it is a slow, heavy mofo compared to the others.
I got all that "at your size, at your weight" stuff as well when I was over 300. If you are athletic for your size, the bike weight will make a difference in performance. My lighter carbon road bike is faster than my aluminum road bike and the difference is only a few pounds. The weight of the bike factors in more than the weight of the rider when it comes to performance, in my experience. If you want speed, bike weight is a factor. For me the goal is the lightest bike that is reliable for my weight. At 325, it was the bikes I listed above.