I've taken one hog with a 20ga slug.
It was with the Remington 5/8oz "Express" as sold by Walmart.
The ~250lb boar was hit in the right shoulder broadside at approx. 40yds. The pig was dropped at the shot as I broke both sholders.
The slug was recovered from the skin on the far side. It looks like a lead "washer" with a hole in the center where the hollow point opened up and stretched on expansion. The wound cavity looked like a "cookie" cutter went through him. The pig struggled to get up for 10-15sec. while I approached it. It was dead when I reached it, however.
With reasonable shot placement and ranges (under 100yds), the Foster type slugs are acceptable. Remember however that they are a poor ballistic shape and lose velocity rapidly and hence, energy.
From what some of my in-laws tell me, (who shoot a lot of deer on dog-hunts near Mobile, AL) there is little difference between the performance of the 20ga and 12ga. Especially with the newer sabot type slugs.
I typically hunt small game these days with a 20ga shotgun (if not hunting with a rimfire), so a pig is a target of "opportunity". Hence, I carry a few slugs with me and previous target the gun to be sure where it prints. Hence, it's what I had at the time.