Was on the fence, but the consensus here seems to be really positive
1) I read the optimization is really good. How does it compare to MGS V/Fox Engine? If I can play TPP on high at a decent frame rate, I should be be to play this okay?
2) Does the game fall into that Ubi-style open-world trap, or does the world feel lively and interesting to explore?
It's smoother than MGSV. If you are running MGSV well, you are fine with MM.
Second question is a bit complicated. It does use the same formula Ubisoft open world games use, but it does some things better. For instance, you are not collecting things that have little to no practical use (even money is fairly useless in AC games, for instance). You will be mostly collecting scrap (which is essential for upgrades you need), water, fuel and ammo. All of them are usefull, though water, ammo and fuel comes more naturally with progression and you don't have to go out of your way to find it.
Here is the kicker though: while in other games grinding hurts the pacing, narrative and immersion, it's the opposite in Mad Max. Grinding for resources actually enhances the world building because it totally fits the tone of the game. A Mad Max game with no scavenging would feel off.
In the end though, it depends on how cynical you can be towards all this. I think it fits the game well, and the level design is good enough to keep things fresh. But there's no denying its the same formula.