Andy_K 

Road bike geometry is a bit different, but part of the difference is that road bike geometry is designed to be used with drop bars. If you put a flat bar on a road bike, as a general rule, the geometry ends up being completely wrong.

Road bike geometry is often geared toward more agile cornering, but for commuting that's generally an undesirable trade-off versus stability. The only other thing road bike geometry does to increase speed is it puts the rider into a more tucked position to improve aerodynamics. Using a flat bar works against that, and my experience has been that as my weight moves north of 200 pounds and my age increases beyond 40 I find myself wanting a more and more upright position. The speed you gain isn't nearly worth the trade-off in comfort.

The biggest thing you can do to gain speed is trade the knobby tires for slicks. Better quality tires add even more speed.

If you stick to tires that are 700x32 or bigger (26x1.6 if you go for small wheels) and don't ride with your elbows locked, your wrists will be safe from curbs. Knees and elbows are way better shock absorbers than any suspension fork on the market, and cheap suspension forks suck your speed.

I'd suggest you make a trip to a well-stocked bike shop and test ride something like a Trek FX or Canondale Bad Boy (both of which are more or less what you've described, minus the suspension fork). After that you can decide whether or not this is the type of bike you're looking for. You can probably even ride something similar with a suspension fork for comparison.