flat foot shifting will get the job done faster, but the bottom line is that it will wear out your clutch faster. how much faster? who the heck knows...=d the car was obviously designed to handle the rough shifting without breaking, but breaking and wearing out faster are two different things. if you're willing to hasten your clutch replacement in exchange for some fun, then by all means, go for it! if you're into babying your car, then FFS realizing that you're putting extra wear on things.
also, something to keep in mind: if you FFS, you will likely cause tire spin in the lower gears, so that's not necessarily a good thing for going fast. i, personally, prefer just to stick with shifting as quickly as i can the good old fashioned way, because FFS is just too much of a double-edged sword for my tastes, and i really don't give a rat's ass about breaking the 1/4 mile a few 10ths of a sec faster...