I doubt using a pistol and an SMG will make you go faster. The game isn't realistic in that sense. Then again maybe you can't even use the lower end weapons.
I like light to the point where I don't like having heavies on my team. It's the principle. Sniping out of a helicopter is harder than fighting from a skyscraqer.
Light-only should be a gametype because all of us hate everything that is not us and we are meant for each other (semi-intended sarcasm, but I still would love to see a light only gametype).
My point was that if a heavy can use all weapon types, and a gatling gun does the same damage as a smg, what is the point of using a gatling gun, which is notoriously inaccurate in shooters, when I can use a smg that by the look of the videos has pin point accuracy? The slow you down part comes from the body type. There is no point in playing a heavy if the trade off is two more hits to die vs. limited SMART movement, IMO that is not good balancing. There has to be an advantage to playing a heavy beyond a slightly bigger health bar, and because heavies can use big guns its safe to say that the advantage is in the guns.
I also you are wrong in that you have the mentality that heavies are no match for lights so you want a light only game mode. If anything a light only game mode would be boring because of the monotony. Everyone would be using the same paths and all that to get to objectives which would boil down to people having shootouts in high up places, nothing more. If everyone moves with the same speed and agility that takes away the advantage of moving with speed and agility.