I think generally the core iron will saturate before you reach the limit of the magnets. In a coreless design, there won't be saturation, but you are then thermally limited by the copper. The torque will be function of the flux coupled between the rotor and stator, so both contribute.
If the magnets are "too strong" you will have increased eddy current losses. Magnets are expensive too, so most designs avoid using more than necessary.
For a given amount of power input, the flux you get from a copper coil will be much greater with a core, until you reach saturation.
"One test is worth a thousand opinions"