So, how many of the "don't bother with it" experts have done anything with theirs, or have even owned one?
The PerfectUnion Forum site was "off" for a few hours recently, but it should be accessible now. The first link given above should work just fine.
Re the "POS" barrel: the earlier models had a barrel known for being rather "whippy," particularly as sustained fire heated up the barrel. Even something as low-budget/low-tech as a good flash hider/muzzle brake can start to correct that situation. As you already noted, a shorter barrel (brought back to legal length with a permanent FH/MB) will make a difference, too.
Cryo-treatment of the barrel (and receiver) might run about $35 plus shipping, and can make a nice reduction in group size.
A trigger job is not a lot of bucks, either, and often turns out to surprise even those who were already expecting an improvement from it. Stock bedding is also a Good Thing™, and for maybe the same price as the trigger job, at least from the one 'smith I'm most familiar with. You might be more adventurous about a DIY project for that than I would be.
The ammo advice above is good. The Mini started out with a 1:10 twist rate, went to a 1:7 for a few years, and has been a 1:9 since 1995. I think I've seen at least one post where somebody said their Mini seemed to like 40 gr. and 62 gr. (or heavier) okay, but not so much the most common 55 gr. stuff.