Say the WBA does not strip Kalambay in '89 and McCallum goes after Nunn for the IBF title instead of fighing Graham for the vacant WBA title. I think Nunn would outslick him for a UD, probably like 8-4.
Assuming both are at their best, then a solid UD for Nunn. Puncher’s chance for McCallum. The Watson fight aside, I don’t think McCallum was ever as dominant or impressive at 160 as he’d been at 154. Nunn, on the other hand, was a huge Middleweight and one of the great talents in that division’s history. Style-wise he’s the last thing McCallum needs; elusive, mobile and as fast as greased lightning. Even if McCallum cuts down the range, which he’d need to do, it’s not as if Nunn didn’t know how to bang hard to the body on the inside. Can’t rule out McCallum doing a Toney on Nunn if ‘Second To’ has a bit of an off-night or mental lapse, but if the Nunn of the Tate or Roldan fights shows up, then I think he wins on the cards in decent style. Kalambay showed everyone the way to beat McCallum at 160, and I think this one might have panned out pretty similarly.
A 12 rd distance favors Nunn, but I don't think he looked that impressive over 12. Maybe those were off nights for him or maybe he had trouble moving and punching like he did for 12 rds. On the other hand, you can argue that McCallum was easily handled by Kalambay in their first fight and had a pretty tough fight with Graham (I don't think it was close, though), who was a bit of a smaller version of Nunn. Hard to say really. You can say that Mike would do better than guys like Barkley and Starling, but you can also say the same with Nunn and Kalambay and Graham.