It's nice to get as stiff a subfloor as possible, but in fact, the MINIMUM TCNA guidelines for a tiled floor over 16" OC joists is 5/8" with T&G (or blocked seams). More is nice, and is required if the tile is natural stone (second layer a minimum of 3/8"). Many pros would not even consider laying tile over 5/8" (except a low-cost new house, or a flip, maybe), but it will hold up if that layer is properly installed and in good condition. It's always nice to get a bit over the minimum, and I'd not have any problems installing over a good layer of 3/4".
I'd have to look it up, but for flatness, I think the standard is 1/8" in 10'.
But, subflooring is only one part of the equation...the joists must also meet the minimum (preferably better) deflection standard of L/360 for ceramic (porcelain is a type of ceramic and has the same requirements). If that's deficient, no amount of subflooring will provide a reliable result.