If you aren't sure of the year SLG that you currently have, please post a picture so that we can help determine that for you
Nevertheless, the upgrades for the SLG include:
-Grips (Ultralite frame only; I am not aware for any grips for the other SLG frames)
-Triggers (Includes many brands; triggers for SLG are interchangeable excluding UL version)
-Feednecks (DM/PM interchangeable)
-Barrels (Autococker Threaded)
-Board (Few brands; board for SLG should be interchangeable excluding UL version
-Inline Regulator/HPR (Dye proprietary threading; Hyper3 interchangeable -- VA converter available for use of regular HPR threads)
-ASA Assembly (MANY brands; self-explanatory)
-Detents (Kila Magnetic)
-Eyes (Virtue)
-Back Cap (New-Designz)
As far as "must-have" upgrades..other than an aluminum clamping feedneck, an on/off ASA, and maybe a new trigger (if you have a plastic one), then you are pretty much set. You simply are not going to get the bang for your buck when upgrading an SLG because most of its upgrades are not going to make a difference in performance, unless of course we are talking about a new bolt or something along those lines. The stock barrel is fine, because "over-boring" seems to be the trend these days, meaning that your barrel bore size is a couple of sizes larger than the paintball. The stock barrel on the SLG is a .690-691 size, so that pretty much accommodates the broad majority if not all of the paintball sizes that are manufactured today. Furthermore, the stock barrel is the same barrel that Proto had offered a few years back as an upgrade for other marker platforms, so you essentially have an upgraded barrel right now, but as stock. The only upgrades that I would look into would be to modify your existing parts (i.e., polish your internals, change the Orings to looser fit HNBR-70's, spring mod, etc). Now regarding the feedneck, they are going to be pretty much the same, but I would play it safe and go with Dye/Proto. Probably the reason why you aren't finding that many SLG feednecks is because the feednecks aren't marketed toward the SLG's. Here's why -- the SLG is compatible with DM/PM feednecks!
*please feel free to correct me on anything that I have stated should it be incorrect, thanks!*
-Jarred