I shoot a marauder in 177 shooting the nsa 15.5 slugs at 830 FPS, it holds one inch groups at 100 yds.
Those NSA markings are very similar to those of my standard twist poly barrel(as in how much of the slugs are in contact with the barrel). The Piledrivers tend to be undersized I think. I've not measured any, but I've read many accounts of them fitting loosely in barrels, fwiw.
I've gotten regular pellets of about that weight and greater length to shoot well in most .177's...so it wouldn't be a barrel twist issue.
Taso, to bad you didn't live closer.. Willow Slough in Indiana has a nice range and 100 yards.. And I have a 100 yard range across the street that a neighbor let me cut in a overgrown paintball field.. I used to live in Frankfort..