RF modulated video and audio is the "lowest common denominator" of quality. Depending on the quality of the modulator, it should match what you're getting now - after all, the Tivo is using an internal RF modulator to create the signal, and all you're doing is moving that process outboard. It's not guaranteed to be equal, but it should be comparable. As for stereo vs. mono, RF can carry stereo and I would expect that your Tivo is outputting stereo now.

Running analog cables can be better quality, but depending on the quality of the TV at the other end and the path that the cables follow to get there the differences may be subtle.

The CE Labs unit you linked to looks a bit more robust than the typical Radio Shack units I've used in the past. I've had mixed results with those - some work just fine, but I've had one die on me.