Isn't there a fisheye straightening tool in Photoshop? (Or something?)
How good is it How much does it degrade the image? Can you get close?
I've got an 8-15mm, but my old PS crashed and I haven't bought a new one yet, and I really balk at the Creative Cloud blackmail.
I've typically applied the Canon 15mm fisheye profile in LR to defish the 8-15 and adjust as needed. It stretches out the fisheye projection to the edges and corners, and the further away from the center the pixels are the worse the stretching. You can crop the defished imaged to eliminate the worst of the artifacts, but then you're sacrificing AOV. Images with geometric patterns (think cabinets, floors and ceilings) are easier to interpolate and look OK, but for details (landscapes), it'll be pretty poor.
The very cheap Plugin FisheyeHemi is a fantastic tool for defishing fisheye images, it doesn't do full rectilinear corrections so doesn't have the edge projection issues a simple remap can introduce, it uses a more complicated remapping algorithm and the pixel level IQ remains very high.
Have you seen this guys method of defishing using FisheyeHemi? He changes the canvas size for a landscape shot into a portrait shot to run it through Hemi again so that it corrects both axis.
The FisheyeHemi stuff is near the end of the article.