The stock shifter is perfectly fine as is. I have my car for 22 months now, and still bone stock, except for a Dinan chip.
I once seriousely considered a short shifter and talked with Greg regarding his take on the subject. He told me that in stock form, the M5 shifter is probably 'shorter' and less sloppy than that in his 996tt.
With this input, I was content with the M5 shifter until......
December 4, 2002, I bought a Miata with a short shifter. A REAL SHORT shifter (3"-4" total!!). The action is soooo precisce, every shift is just a tiny 'snick', and the lowered shift knob position induced a much sportier feel.
It's gotton to the point that now I feel the M5 shift knob is way too tall.
It's not the shift action, just the hand position. So I'm tempted by a SSK just to lower the shift knob height. I'm sure eventually the shorter throw will feel great too.
Another point, I have enclosed garage both at home and at work, so I'd never have a real cold start, so cold-weather notchiness associated with the SSK should not be an issue for me.
I guess like Ray says, the SSK is something you need to experience to appreciate. A good parallel may be a garage door opener. I lived years without it, opening/closing the garage door manually. Then I got one, and wondered what was I thinking before.