This is one of those rare things I disagree with Denny about. It definitely carbs faster going through bottom post, especially if you use a higher pressure and/or shake. Also, the more head space the faster the beer will carbonate.
Nothing beats a diffusion stone though. I can carbonate a 30 bbl batch of beer in a few hours. They sell them for corny kegs. You could probably carb 5 gallons in less than 30 minutes with one.