My response to the first question would be yes.  The Renesis exhaust back pressure with the turbo at 23" MAP was about 7 psia.  If there is an effect of this back pressure I was surprised at how small it was.

The intake manifold for the 13B,  NA, TC, SC, was the stock intake and throttle body as found in the 1986 RX7 car.  The intake manifold for the NA Renesis is shown in the attached photos. The same intake manifold was used for the TC Renesis except that the runners were as short as possible while still extending 4" into the plenum.  The Renesis is a model year 2011 which only was produced as a 6 port intake configuration.  The intake manifold in the photo allowed the auxiliary ports to be open or closed.  At my elevation with the NA WOT RPM of about 5300, the engine makes more power with the auxiliary intake ports closed so that is how the data was generated.

The low HP at NA WOT is a result of the elevation limiting the MAP to 23" Hg.  If the results are converted to standard day sea level and corrected for RPM assuming a flat torque curve, the results agree closely with Mazda's HP claims.