As the root/hub radius and tip radius are increased together at the same rate the effect is that of moving a blade of constant length further out from the axis of rotation. As the blade position is moved outwards from the centre, the maximum CP decreases and the inflow velocity that it corresponds to increases, however, at higher inflow velocities, the drop off in CP becomes less severe resulting in higher CP values at higher velocities but operating an MCT at these velocities would be unlikely. Once again, increasing tip radius and maintaining rotational velocity will result in increased tip velocities and as such, cavitation will become an issue beyond a certain point, depending on your rotational speed and aerofoil choice.
