JBL uses these "RBI waveguides" on their prosound line these days
I decided to give this a try on my system.
Here's the frequency response and distortion of the 8NDL51 in my small sealed box
Here's the polar response of the woofer. (this is with no waveguide.)
Here's the horizontal polars of the woofer, using my crude approximation of JBL's RBI waveguide.
yowzers, this is impressive performance.
Look at that! The RBI basically makes this 8" woofer behave as if it was just 2" in diameter. The off-axis response is virtually identical to the on-axis response, all the way to 7000Hz!
There's no free lunch though. Here's the vertical polars. To me, it looks like the RBI makes a HUGE improvement in the horizontal polars, but it also has a negative impact on the vertical polars.
I have a feeling you could probably come up with a compromise between the two by making the aperture shape closer to circular, instead of elliptical.