Anderton
I don't see a need for mono-to-stereo. I record mono sources all the time (vocals, guitar) with mono tracks, insert stereo plugs, get stereo outputs. Problem (if in fact there was one!) solved.
Me as well Craig.
As a matter of fact Olpal, I don't even have any of the mono plugs showing in my plugin manager. I HATE all the ones that Waves and UAD give me. But I've never had a problem using stereo plugs in mono tracks. At the end of the day, you're not going to get "stereo compression" or "eq". The only time it becomes something that would be stereo is when you use effects like delay or a chorus....phaser, reverb, you get the idea. But stuff like wah, over-drive/distortion, eq, compression, filters, and anything that is more a processing effect as opposed to an enhancement effect.....you won't see any issues just using the stereo versions in my opinion.
Now that said, you DO hear a difference with the new UAD Fairchild tube stuff. The mono version (which is also stereo) of the 660 to me sounds different than the 670 which is full stereo. Though they try to push one as "mono" they are both stereo yet sound like two different beasts. But for the most part, I don't think you'll ever run into any issues by just staying stereo unless you have some sort of weird routing need.
-Danny