We don't have the kind of on court, off court ratings for Jerry West that we do for more recent players. but with some of the wonderful additions to Basketball Reference, we can at least get a rough estimate. Basketball Reference now has scans of newspaper box scores for every NBA game, ever. The data in these box scores are limited though. We usually get how many points, field goals, and free throws a player has, but attempts are usually not there, so if you want to know if West held Robertson to a below average FG%, it's not there. But just knowing which games a player played in can get you some interesting results.
Jerry West missed a decent amount of games every year. I focused on 1962-63, and 1966-7 to 1968-9. He missed 25, 15, 31, and 21 games those years.
1962-63: Without West, Lakers were 11-14, being outscored 111.1 to 112.8. With West, they were 42-13, scoring 117.5 and allowing 112.2.
1966-67: Without West, Lakers were 4-11, being outscored 116.6 to 121.2. With West, they were 32-33, scoring 121.1 and allowing 120.
1967-68: Without West, Lakers are 19-12, but only outscore opponents 114.6 to 114.4. With West the record is similar (33-18) but they outscore by 125.2 to 116.3
1968-69: Wilt's first year with Lakers, BTW. Without Jerry: 12-9, 108.7 to 108.4 Pts/Oppts. With Jerry: 43-18, outscore opponents 113.5 to 108.
Jerry's teams were generally 5-9 points better in games that he played. This is equivalent to about an 8-10 advantage per 48 minutes. This figure would be among the best in the NBA today, but a few players (Lebron, Chris Paul, Garnett) have been above that level the last 2 years, according to 82games.com.
Over those 4 years the Lakers were a .500 team without West (46-46) but 150-82 when he played.
Wilt Chamberlain presents a tougher challenge, and that would be true even if the NBA in the 1960's tracked plus minus to the same degree they do today. Wilt didn't miss many games, and he almost never came off the court (played more than 48 minutes per game in some years thanks to OT). But we do have some indication of his impact, thanks to one year where he missed 70 games, and two trades.
In 1970-71 Wilt missed all but the final 12 games of the season. Lakers were 44-26, outscoring opponents 116.4 to 111.9. Then Wilt came back, the team finished 4-8, being outscored 105.4 to 110.3. It's not totally fair though, as Jerry West missed all of those games, and the team was probably more concerned about getting Wilt in shape for the playoffs than winning out the regular season.
Wilt was traded in mid season 1965, and for 3 fairly inconsequential players. Here's his impact on the Warriors (who traded him) and the Sixers (who got him):
Warriors were 11-33, outscored 108-113.3. How this is possible with a player like Wilt I don't know. Afterwards they went into full tank mode, went 6-30, and were outscored 103.1 to 110.4.
Sixers were 22-23 at the trade, outscored 111.4 to 111.9. After the trade they were slightly better, outscoring 113.9 to 113.7 and going 18-17.
Wilt's trade from Philly to LA is more problematic, as the Sixers got two decent though not great starting players in Darrell Imhoff and Archie Clark. Lakers improved by 3 games, and Philly declined by 7.
Wilt's estimated impact per 48 minutes from these with/without situations isn't that much, between 1-4 points. That's excluding his 12 games with the 71 Lakers. Wilt was part of the two greatest single season teams before Jordan's 1996 Bulls, and when everything was working right between teammates and coaches I'm sure his impact was as great as any player ever. But for too much of his career it's hard to see evidence that his dominant stats did much to impact wins and losses.
On the Wilt-Russell debates I was always on the Wilt side from the first day I learned to add up points, rebounds, and assists. But I don't think that's right anymore, and I think a legitimate question is whether Wilt was the second best player of his generation after Russell, or whether it was Jerry West.