I can't speak first hand on the pre 580 sreies mini's but the newer tapered barrel 580's seem pretty accurate. I never noticed any difference in accuracy when the barrel heats up with my 580 mini. IMO the strut is not needed.
Do you have any documentation proving this?Newer thick barrels are cryo treated by Ruger. The thin barrels were not and did string shots as they warmed up. Newer barrels don't string as they warm up.