Apparently, the FJ coils work but the bump stops need a little modification....
If I would have spotted this option before I bought the OME's (abet for $100, a good deal), I may have had a go at trying the FJ's--- they appear way cheaper. As I tow much more than off-road, having firm rear springs would be a good idea.
As far as ride is concerned, the OME's are a thicker coil than stock but are a progressive spring, so they give, but as they compress they become stiffer. According to the posts from 2012, it appears the FJ's behave similarly to the OME's (soft but don't sag when loaded), but it's impossible to tell which would give a more comfortable ride (not to mention the type of shock you use) unless you have the trucks side by side!
The important thing is they don't sag or bottom out with rough terrain. I believe the jarring ride I felt, before I replaced mine, was due to the spring bottoming out. My stock springs were really saggy-- not to mention the shot shocks.