I'll just note that while not often done, Multiple Master as a format allowed intermediate masters and did not require masters in the corners of the design space, as well as glyph-specific design shifts in the space.
But as usually implemented, it did require these things.
While I wouldn't say that Ben is "wrong," I would say that my experiences and results tend to lead me closer to Georg's line of thinking.
Given the design of Cantarell, you can go as light as you like, and a step or two bolder, before you start to need to do anything special for optical compensations with an extra master. At first it would just be for a few glyphs such as a and e. If you go a lot bolder, you'd need to do a whole master.
You should try starting with keeping the Regular as a master, and doing the lightest and boldest weights. Then see how different it looks (and whether it's a problem) if you drop the Regular master.
That said, in general I would just use two masters, and do corrections at the heaviest end as needed (per Georg's comments). The reason being that for further development, changes and extension, it's just a lot less work if you have two masters instead of three.
And as a bonus, the final file size would be smaller. I know you say that's not a priority, but it can't be a *bad* thing, right?