A tampa with the speed sling and taped magazines has the fastest weapons swap speed and reload speed amongst the SMGs and probably all pistols as well.
Yeah, I played around with some SMG's too and found the Tampa to be better than expected because you can buff the reload tremendously. I tend to run out of bullets so the reload really helps.
I still prefer the Carb-9 but being a light operative, I spend a lot of time running, gunning and turning corners and the carb with a drum help a lot, spray a lot, hurt a lot, don't care about accuracy. If I had better aim, I guess some of the other SMG's would be better, but the Tampa clearly wins on all-out reload speed.
The time you spend in-game, not being able to shoot counts for something.
I guess the carb and kross just fit the most preferred playstyles out there.