The claimed fuel economy improvement in the Renesis was not really found at all.  In fact, most RX-8's got worse fuel economy than non-turbo RX-7's (and quite a few of the turbo ones).  Partly this is because the RX-8 is a heavier car than the RX-7, but basically the Renesis is just not all that efficient in real-world conditions.  Maybe in the lab.

Because of the generally disappointing performance of the Renesis engine - which was less efficient, less reliable, and less powerful than originally advertised, and on two of those three categories also compares unfavorably to the (then) 40 year old 13B design, I have somewhat of a skeptical attitude toward the 16X.

Certainly I think the concept - increase the eccentricity for more torque and less revs - makes sense.  The existing Renesis was limited in RPM more by the available transmissions than by its own capabilities, and thermodynamic efficiency (already the weak point of the rotary design) decreases at high RPM.  Even with a "long stroke" design the 16X will still be a high revving, free wheeling engine and should retain most of the rotary character.

But Mazda hasn't produced a genuinely good rotary engine in 20 years.  I admire their dedication, but they really need to start getting it right.