I going to build a 10.5", .300 BO pistol. i have a complete lower with a SBA3 brace with a carbine buffer and spring. I know that the PSA kits come with the carbine spring and buffer, but is this optimal? This pistol will not be suppressed now, but hopefully it will be down the road.

That’s probably optimal if you plan on shooting supers and subs

My 9.5" Blk I have has a standard buffer and spring, but I also have a adjustable gas block and it is not wide open.

Carbine buffer is fine.

Thanks for answers. Order is in for the upper.

Straight from AAC themselves..
There is no right answer for different manufacturers and barrel lengths. The AAC 16" needs a carbine buffer to run subs without a silencer, but their 9" uses an H2. We recommend a H2 buffer for our 8.2" 300 BLK barrel, but you can use an H if running without a silencer or only subs.

IMPORTANT NOTES:
-The 9" and 12.5" AAC 300BLK Uppers are designed for use with a carbine receiver extension, standard GI Buffer Spring, and an H2 Buffer.
-The 16" AAC 300BLK Upper is designed for use with a carbine receiver extension, standard GI Buffer Spring, and a CARBINE Buffer

I'm sure you will be fine with carbine/carbine but I run and H2 with carbine unsuppressed and it functions perfectly with all types of ammo.

It all depends and you are not married to one specific buffer weight.  You can always test different weights if you have issues or swap for different loads.  Optimal will be you testing and seeing what works.

No issues whatsoever with 7.5 and 16 inch BO shooting 145-220 gr loads with standard carbine buffer and spring. Definitely some fine tuning that can be done with specific loads etc.