1980: The Lakers would have won regardless of which division. Kareem/Magic/Wilkes/Nixon were too dominant.
1981: The Lakers weren't great that season, and lost to Houston anyway. Magic was hurt most of the year, and may not have been fully healthy for the playoffs. Celtics/Sixers were 1-2 that season, and it wasn't particularly close to #3.
1982: I do think the Lakers may have lucked out a bit in that Philly was pretty much battered after their 7-game series with Boston. I think the Celtics or the Sixers may have beaten the Lakers in a Conference matchup. Celtics under achieved a bit against Philly, and then Tiny got hurt.
1983: Philly was winning no matter what. Moses Malone was in his prime and gave Kareem fits.
1984: Finals matchup was perfect. 2 best teams. Had the Lakers been in the East, it's entirely possible the Knicks or Sixers could have given them a scare as well. The Knicks extended the Celtics to 7 games, and the Sixers under achieved in a bizarre matchup against the Nets. This is one season where had the Lakers been in the East, I would bet against them making the Finals.
1985: Celtics were worn down by the playoffs, and the Lakers probably would have beaten any of the other Eastern teams. The Celtics handled Philly pretty easily in the ECF.
1986: Celtics were the most dominant and complete team. Beat the Lakers pretty convincingly twice, once without McHale.
1987: It's probably still the Lakers. Pistons and Bucks gave a broken down Celtics team a lot of trouble, and so could have done the same for the Lakers as well. The Celtics got a bit lucky in that final game against the Pistons. But I don't think the Pistons would have been ready to beat the Lakers, who were at another level that season.
1988: The Lakers did beat the Pistons in 7 in the Finals, so I will assume the same would have happened in the ECF. But it was close; final 2 games were decided by a total of 4 points.
1989: Noone was stopping the Pistons that year. Lakers would not have come out of the East.
1990: The Lakers record was a bit inflated that year, and they lost to the Suns. I doubt they would have gotten past the Pistons. Or the Bulls for that matter, and even Barkley's Sixers could have given them trouble.
1991: The Bulls were not going to lose.
And that completes the Showtime Era, which covered 12 seasons, 9 trips to the Finals, and 5 Trophies. Had they played in the East, I group them as follows:
Would have won anyway: 1980, 1985, 1987
Would definitely not have reached Finals: 1983, 1989, 1991.
Too close to call: 1982, 1988
Would have been difficult: 1984
Would still have not reached the Finals: 1981, 1986, 1990
I feel comfortable in saying there's 3 trips to the Finals that would not have happened had the Lakers been in the East, but none of those shortened playoff trips would have resulted in less titles. If we assume they go 2-1 in the "too close to call" and "difficult" categories, they probably have 1 less title.
So the bottom line is likely 5-6 trips to the Finals and 4 Titles. But it's possible that the Celtics could have had 1 less title in the process.