About watch and caustics - these are reflective caustics. They should usually render pretty fast. Refractive ones - that's a different story. ;)
Did some test :- ) Not at all from this experience (or all past tries). I only rendered that little watch piece, so basically 500x500px, not a lot. Using default MSI 20 (anything else wouldn't clear at all).
1000 (!!) passes later, using default 16/2 settings GI/AA, it's still noisy. And this is basically no-GI render, it's HDRi without any geometry around the scene, super simple to render. Everything else was clean at 10 passes :- ).
Caustics, as they are, are basically useless. This direct approach isn't imho feasible at the moment.