Dave,
90 yards is pretty good with a Foster style slug. I think Bigbulls has a great point about the energy being gone by the time that it hit. I've only used slugs on closer ranges, and although I've never recovered the slug afterwards, the extra large exit wounds showed pretty good expansion. FTR, I've used Federal foster style and sabots. My buddy shot a 5 point Saturday at 75 yards, with a Federal sabot, and it expanded very well, leaving a huge exit. Only probl;em witht the sabots is cost. $9- $13 a box (5 shots!), cost a fortune to just sight in.
Charlie