I'd add Edberg as well, as he lost in a 5-set French Open final.
All legacies would be enhanced, obviously, some more than others.
Lendl (adding Wimbledon) would be a 9-time Slam champ with a career Grand Slam.
Borg (US Open) would be a 12-time Slam champ with wins on all surfaces
Mac (French Open) would be an 8-time Slam champ with wins on all surfaces.
Edberg (French Open) would be a 7-time Slam champ with a career Grand Slam. I added Edberg and took away Becker since Edberg was actually in a French Open final.
Courier (Wimbledon) would be a 5-time Slam champ with wins on all surfaces.
Not sure if these players should be on the list, since they never made the final in a "missing piece." But, anyway:
Jimmy Connors (French Open) would be a 9-time Slam champ with a career Grand Slam.
Becker (French Open) would be a 7-time Slam champ with a career Grand Slam.
Wilander (Wimbledon) would be an 8-time Slam champ with a career Grand Slam.
Not sure if you're asking what would happen if ALL of these things happened, or if we're going down the list and asking what would happen to this ONE player's legacy if he was the only one on the list to add a Slam.
If they ALL have the added Slam, not much changes in relative terms since they're all filling in missing pieces (some to achieve career Grand Slams, some to get 3 of the 4 and wins on all surfaces.)
If just one player has the added Slam, I'm not sure. I think Lendl's legacy would benefit the most. For some reason, maybe unfairly, that missing Wimbledon is seen as a more glaring omission for Lendl than the missing Slams are from the other players.