I own a Miata which has a 6spd in it and Ill tell you the gearing is far tighter and often ill find myself simply skipping over gears as they arent exactly needed. The 6th gear isnt a taller gear as you get with many 5 to 6 gear upgrades. The 6speed is slightly more robust, but unless you are planning on pushing 250+ hp through it, you wont see any gains there.
Many people who autoX and otherwise actually swap their 6 speed for a 5 speed.
I maintain a 6 speed box on the M3W would be a great advantage given the narrow power band. I find 1st gear fine and certainly wouldn't one lower, but in town traffic with 30 and 40mph speed limits, spend too much time with the engine at 2400 (too low) or 3500 rpm (too high) when in traffic on a light throttle.
An extra gear that allowed me 3000 rpm when trickling along at such speeds would be great. Better chance of finding just the right gear for a swift overtake manoeuvre as well
