I think the Jays with Bellinger vs Kiermaier would receive more than a single win bump. Re-signing Kiermaier means that Varsho isn't spending full time in center field, and I think this alone costs the team another win at minimum, and potentially as many as 2 full wins given Varsho's relative value in center field defensively vs playing the bulk of his innings in left field again. It would be a tremendous waste of a guy who is potentially MLB's best outfield defender to have him spend another season in a corner outfield spot.
Kiermaier has a 1.1 win projection vs the 2.4 win projection for Bellinger, but unlike Kiermaier who's projection seems very reasonable I think Bellinger has a very good shot to beat his projection. He provided value commensurate to a 5 win player when you prorate his time to a full season of play. It seems unlikely he will be able to completely recreate the success on the field, but let's say he beats his projection to a very reasonable degree and produces a 3 win season compared to the single win season for Kiermaier. Now the split from Bellinger vs Kiermaier is suddenly 2 wins. Add that to the 1-2 wins from playing Varsho in center field for a full season and the team receives a 2 win increase in wins on the lower end, with a really easy path to enjoy a 4 win or more increase in wins between the two players if Varsho continues to be a 4 win center fielder as he has been in his career up to this point and if Bellinger maintains even a modest increase in his offensive gains as compared to the modest projection.