I have one with a 40 degree improved chamber right now, The long action is sort of a happy accident, it just happened that the only standard magazine I had was long action and the receiver had an LA bolt stop in it.
- Short action M+ magazines can handle 2.950", which is about where a 115 DTAC sits with the neck/shoulder junction even with the boattail. No need or ability to go longer than that.
- Mine feeds pretty well out of the LA mag. A short M+ would probably be slightly better to control movement during recoil, but that hasn't been an issue yet.
- The factory 1:8 throat isn't very long, probably the SAAMI throat length. I throated mine quite a bit deeper so now my COAL is 2.890 with the 115 DTAC .060" off.
There's no harm in using LA components, but no benefit either. The M+ magazine will handle the longest a 243 would conceivably be.