"So - initial reactions - the negs are very dense, the stain is extremely brown and prominent, (as compared to mild and weak with exactol lux) they are bitingly sharp (even when taken with the holga!). There is very good shadow and highlights details. I have only scanned the negs so far, but have found the grain to be really heavy (sharp but heavy). The tonality is wonderful though."
HP5+ is a film that stains well with Pyrocat-HD. Indeed, with rotary processing it stains almost too well unless you slow down the rate of rotation a lot. My medium format HP5+ negatives are characterized by a nice brownish/yellow stain, distinctive but not heavy, and slightly coarse grain.
Using the type of agitation you describe, and medium format film, I would not have expected properly exposed negatives to have an extremely brown stain, nor to have really heavy grain. It sounds to me that you overexposed the film by quite a lot, at least one stop and perhaps even more.