Here are some links that might help:

This is the Liphook graph that shows PPID horses have much higher seasonal rise ACTH than normal horses, so that an ACTH of 100 pg/ml means PPID, not a normal seasonal ACTH:

These two links should take you to the Pergolide Database; there are 75 horses whose dose is 3.0to 10.0 mg; and a further 14 with doses above 10.0 mg (as well as the many, many horses with doses up to 3.0 mg)  The first pages all have case histories attached, so one can review and see the factors involved, the diets, the horse's status, etc.  Not much sign of "toxicity" there. (I know I am preaching to the choir!) Merlin is on 16 mg (typo in database, must correct that), and at age 34 moves better than he did at 28, before his initial IR diagnosis (diagnosed PPID at age 29)

The next two links take you to the Proceedings of the 2013 NO Laminitis! Conference.  You can also get these from the ecirhorse.org webpage, but showing that the ECIR Group has proceedings on a reputable veterinary database might help. (unfortunately, none of the papers directly deal with pergolide dosage)

You are so correct - the consequences of not having PPID controlled, and Lyme controlled, are devastating.  It wasn't too very long ago that PPID was virtually a death sentence, or at the least a fairly miserable existence with laminitis every year, because it was just assumed that that is the progression of the condition.  Well, yes, it is the progression of the untreated, or under-treated, condition, but it no longer has to be that way.