I toyed around with getting an SSP for ages, but I was always worried about the length of the ball stud making clearance issues with my CorkSport airbox (the JBR one for exmaple) or actually making shifting worse, which is a common complaint of these things.
I was following an unknown bloke who whipped up his own design on MSF.org and decided to try it out (got the standard 2 plate version) and it is looking like he'll get vendor status there: Designed my own short shift plates - Page 4 - Mazdaspeed Forums
Pretty good - I thought the shift improvement claim would be a bit of snake oil, but I think it actually does help. I think it has a lot to do with the counter weights, it makes me think even on the stock shifter, the counter weight is too heavy.
Here is that guys site if anyone is interested: Adapted Performance Shift hammer Mazdaspeed 3 6 Short Shift Plate | Adapted Performance - Mazdaspeed 3 & 6 Performance Parts