There are big tradeoffs going for the transient perfect designs. The biggest, in my opinion, is the rather poor power response that can result. Move off the design axis, and the transient perfect character disappears due to changing the distance to the acoustic centers of each driver. It's much easier to maintain a smooth off axis response using higher order filters.
Going beyond 4th order acoustic doesn't seem necessary unless pushing drivers close to their limits of usability (metal cones near their breakup modes for example.) My personal favorites are 2nd order 3-ways. They tend to offer seamless transition from driver to driver and allow reasonable control of out of band energy, allowing each driver to perform cleanly and without strain.
Going beyond 4th order acoustic doesn't seem necessary unless pushing drivers close to their limits of usability (metal cones near their breakup modes for example.) My personal favorites are 2nd order 3-ways. They tend to offer seamless transition from driver to driver and allow reasonable control of out of band energy, allowing each driver to perform cleanly and without strain.