Are you using a ASI gas bushing in your 580? That would explain the harder ejection with the .040"
Honestly a .040" bushing is mostly too small for most mini's, if cut to the right size/length.
ASI makes a in between size to fit the old mini's and new mini's. Problem is they take different length gas bushings. ASI's bushings are either too short or too long, short gas blow by, long good seal but risk peening the gas bushing (can be a bear to get out) or cracking the top of the gas block.
Based on feedback a 580 with the flanged barrel should be using a .045"-.050" gas bushing if it's cut correctly.
Also worth noting that the gas bushing acts as a key in keeping the gas block in place.
Would it cause 6" stringing at 50 yards with the wrong size bushing? I doubt it but it sure won't help it, however it may be a connected issue with the gas block, heat and contact pressure on the bottom of the stock/gas block fit.
The gas block assembly is a source of a lot of accuracy problems on the mini. Let me clarify, not just the gas block but the entire set of forces with the op-rod, spring and re-chambering of the semi-auto action. The gas forces start on the barrel through the resistance of the recoil spring as soon as the bullet passes the gas port, even though the op-rod hasn't moved. If you want to know how accurate your mini is start with a solid gas bushing turning it into a single shot that will give you a good baseline.
What series is your other mini?
As far as struts go. I would go with a three point contact, 2 clamps and set screw. It's the 2 clamps that will counter the flex in the barrel.
But my first recommendation would be changing the bushing to get the ejection forces controlled.
If it's ejecting 30-40 feet that is about twice as far as it should be and shows there is way too much gas going into the system, remember too where the gas is vented, into the barrel/chamber area, so more gas = more heat. The choate handguards do a better job of venting the heat than the stock handguards.
