Upgrades consist of:
- Apex Spring Kit and Trigger
- Steel recoil spring assembly (read the thread on this before doing it!)
- Improved (wider) takedown plate from Galloway or NDZ
- A wide array of sights (i.e., sights that fit the M&P also fit the SD-series)
- Talon grip panels
- Two slides available -- black or silver-colored stainless steel
Am I missing anything?
I don't think there is a threaded barrel or a custom barrel for the SD, such as those made by Lone Wolf, Storm Lake, Apex, etc., for M&Ps.
As noted by others in this thread, aftermarket parts for the SD-series are few, compared to Glocks and M&Ps!