Healthy Nika Luffy vs. Kaido would mean:
- Luffy hits much harder from the very beginning, as he combines his attacks with not only internal destruction but barrier and sky splitting Haoshoku
- Luffy can take much more from the very beginning, thus wearing himself down at a slower rate (not only awakened mythical Zoan endurance, but also the ability to mold around blunt force), while also being able to bring himself back with his heart beats.
As a matter of fact, both Kaido and Luffy were on their last legs (luffy literally died before their final fight, also periodically dying during the fight) and in the end, Luffy won against Kaido's strongest move (Blazing Bagua).
You can argue about Kaido vs. Luffy being an either way, with Kaido may or may not edging Luffy out, but as a matter of fact, they are that close.