Here's an example using the meike CPL. In this case used with a EF 16-35IIL, a ef 8-15mm Fisheye L and the mighty ef 11-24mm L.
Curiously, the fisheye has a similar angle of view as the 11-24L (at 11mm) once it is de fished in Lightroom. However...the corners are way more mush than the 11-24L. But the fisheye can go a very lot wider.

/\/\ ef 11-24mm @ 11mm f4

/\/\ ef 11-24mm @ 15mm f4

/\/\ ef 8-15mm fisheye at 8mm F4

/\/\ ef 8-15mm fisheye at 8mm F4, de-fished in Photoshop.

/\/\ ef 8-15mm at 11mm F4

/\/\ ef 8-15mm at 11mm F4 De-fished in Lightroom...which is weird because it's a wider angle ofview to the defished 8mm shot! Something funky is occurring with the Lightroom Fish eye profiles.

/\/\ ef 8-15mm at 14mm F4 defished. This angle of view is very similar to the ef 11-24mm @11mm.

/\/\ ef 8-15mm at 15mm F4 unforected. At 15mm, the fish eye sees a lot wider angle of view than the ef 11-24mm can at 11mm.
All of the above images use a 2 stop CPL, fully polarised and have exactly the same settings applied to each lens, because the filter is a drop in and there for static between lens changes.
The EF 11-24mm lens is a huge and heavy beast...but it is spookily sharp wide open. I have never seen an ultrawide this sharp before. It has dark corners / vignetting wide open at F4. I used to have the original Sigma 12-24mm EX DG lens (backj when it was launched) and it's nearly as wide as the 11-24mm. But this Canon variant is a lot sharper and the corners are usable wide open.
The wee little Canon ef 8-25mm fish eye is also an astonishing little lens. Ignoring the weight and size difference...optically it's a little sweet heart. Very sharp wide open and mind blogglingly wide. Yes it really can do a full 180' view. Defished, it's the widest one can get on the Canon mount. It is rediculously wide...however the corners are smearly and mushy. Looking over some older de fished photos, Adobe seemed to have tampered with tier current profile for the 8-15mm lens and it's a lot more smeary in the corners than it used to be in a previous LightRoom build.