My build - my first - is just about complete.  I'm not running a can, at least not yet.  I'm putting together some mid weight ammo (around 125g), all supersonic. My question: does the buffer spring weight need to be specific to the ammo; would running subsonic ammunition with a can require a different spring weight?

Also, would there need to be any changes to the gas system between running super or subsonic ammunition, can or no can?

I know its not the answer you looking for, but I think its a try it and see.

I have a BCM 9.5" I think, and Larue and Noveske in 12" and 10.5" (or .3) and they all run fine suppressed or not, Supers or Sub. Though I'll admidt, I shoot way more subs than supers as I did not go .300 blk for Super capability.

But I have seen many others need to adjust their spring, buffer and in extreme cases the gas system.

In my experience, no. Not at all.

10.5" 300blk. I do run suppressed, but don't have to.

I didn't do anything differently than any other build I've done. Standard lo-pro gas block, standard BCG, and a Geissele Super42 with H2 buffer. Runs like a clock with supers, subs, suppressed, unsuppressed, or any combo thereof.

Don't overthink it. The cartridge is designed for what you're doing. Put it together and shoot it!

Also, hurry up with the can . 300blk is intended for supressed shooting

Depends.  Depends on the ammo, the gas port, and whatever buffer/spring.  Spring should be fine regardless.  When you add a can you effectively increase dwell time, which can result in gas coming back through the action.  Increasing buffer weight can slow the action, as can reducing the gas.  As said above, you will need to shoot it to find out, and respond accordingly.

Use a standard carbine spring and H2 buffer and you should be fine with super and sub.

I have a 10.5" PSA pistol w/pistol gas system and a standard carbine buffer (3.1oz) and spring. I have a can on-order and it's expected to arrive any day now, waiting on the ATF. In preparation for the can I loaded up 50 rounds of 220 gr subsonic ammo and ran some through it this morning. They ran ok but didn't always lock back the bolt on last round.  Lots of smoke. Accuracy at 50 yards was ok with open sights. Recoil was much less and more of a push.

Load:
220gr lead polymer coated (Blue Bullets)
7.2 gr Lil' gun
Win Small rifle magnum primers.

I've read Lil' Gun doesn't always reliably cycle in pistol platforms with sub loads so I'll try more powders as I can.

As far as supersonic loads, I run 110gr vmax, 125 gr spitzer and 147gr FMJ's without so much as a hiccup. I think your standard carbine spring and buffer will cycle all factory ammo. Check your ejection pattern and if it's at 3-4 o'clock you are good. If it's at 1 or 2 O'clock, use and H1 or and H2 buffer. You can buy tungsten weights on Amazon for $30 delivered and adding weight to the buffer is simple.  

diy 10.5" here, standard carbine lower with SBA3.  pistol gas, yhm clamp on gas block.  no issues at all.  zero.  eats anything I put through it.  subs, supers, all the big name ammo manufacturers, doesn't matter.  

not suppressed yet.  

this is what 300blk was designed to do.