Never fired a 20 ga with slugs and only here an there in a 12ga, nor killed any deer with em.
One thing I tell you from what experience I have with them is If you have 3" chambers get 3" slugs and forget the 2 3/4. You will get your best accuracy using the right length shell for your chamber.Ran into that with my uncle's Win 1200 2 3/4" gun,only slug barrel we could find was for a 1300 and 3" chambered smooth bore.Night an day differnce between it and his 1400 an my 870 both with 2 3/4 chambered barrels.
Read an article recentaly on the Savage bolt 20 ga,don't recall the model # but they droped there 12 ga going with this one. Article was in either "Shooting Times" I beleive or possible "Rifle Shooter" by Richard Metcaff (SP) he came to the same conculsion as we did.
IF I had a Fully Rifle Barrel I would reach for the Federal offerings with Barnes Tipped 1st all other Barnes next,followed by the Wincheters and I do think they offer a 20ga XP3.
A freind of mine and some other of his family memebers have 20ga Rem 870's set up with the Remington Cooper Solid's,they like em but I haven't got any detailed on game reports other than they work.
Good Luck