Now that I have a lathe, I would probably make the barrel extension out of aluminum and turn a nice aluminum bushing to fit it to the barrel. TiG it all together, pin it, and float a puddle over the pins. That would save a little weight, even though it isn't very heavy as it is. Other than that, the only thing I can think of that I'd change is simply to add a good quality red dot sight, maybe a Holosun unit.
With the suppressor and 500-grain subsonic loads, my rifle kicks like an 870 12-gauge with high-brass 1-1/4 oz. slugs, I never shot it without the suppressor, and wouldn't want to! I'd keep whatever brake it comes with, and be aware that the threads are probably proprietary unless they're Tromix (Tromix uses 30-cal threads, 5/8x24). The tank brake is on there for a reason, these things in an AR platform will wallop you pretty hard.
I did a lot of research before buying this and found no real surprises along the way. I wouldn't buy any parts other than Tromix or Teppo Jutsu, the reamer prints are still proprietary and as far as I know the other outfits like Radical and CMMG aren't necessarily making proper chambers. Doing this will save you a lot of headaches later. If you get a Tromix upper receiver it will be ready to go, if not, I can give you dimensions and such to modify the ejection port so the cover will still latch properly yet never give you a jam. Marty posted some dimensional drawings of the port mods on arfcom, you can go off of that but it isn't necessary to cut the top of the opening out as much as he does. Other advice I have is to buy some Lancer magazines, nothing else works as well for me (Lancer sucks for everything else BUT the SOCOM, IMO, nothing but troubles with the feed lips due to a very unfortunate engineering oversight on Lancer's part, but that's another story), and buy a Tromix case gauge for setting up and checking your loads.....the SOCOM set up to shoot cast bullets will have benchrest match chamber/cartridge tolerances and it being an AR-15 with weak-assed self-loading and bolt camming action you'll want to make certain that every round will chamber after you load them. The cutaway case gauge is expensive, but priceless for seeing how things fit and for establishing bullet seating depth, crimp process, and for observing interferences. I use Lee dies but had to make a larger and longer expander plug for the PTE die in order to use cast bullets. I also honed the neck of the sizing die a few thousandths, not easy since it is a blind hole, but necessary for cast. The cartridge cannot stand to have the neck and body not concentric, so the best way to keep them that way is not to over-size the necks in the first place.
If you dive into this, start a build thread and put up some photos if you can, I think you'll love the caliber and the platform, I know I do. I'm already wanting a second one in pistol configuration now that there are some much better arm braces available.