Before you go and by an adapter check the POD owners manual to see if the the left output jack is also the mono output jack--this is pretty much industry standard anymore. As for diminished performance, as stated before, you will be loosing the spaciousness of the stereo field. This will be most noticible with some of your delayed settings, flange effects and your chorus effects when these are set up in stereo. Now, I don't have a POD (yet) but most of my rack effects sound pretty decent in mono if I take the time to tweak them just right. But I don't do this often because I like my guitar in stereo--at the least, my lead stuff. So I will record alot of my mono stuff (Bass, rhythm guitar and keys and then bounce them together onto one track and then record my lead onto two tracks. I then will master all of this onto VHS in stereo and record this onto two tracks of a four track and use the other two for vocals. Oh yeah, often, if I want my rhythm in stereo, I will record this with my drum machine, use track 3 for bass and track 4 for keys, master the rhythm tracks on VHS and then record thes on two tracks and then place my lead, master again, instruments on two tracks have two left for vocals. I know this is more than you asked for, but I could see where you next questions would go (or so I guessed). Also, everyone (especially in the analog world) seems scared to bounce or do reduction mixes (rhythm tracks as I call them). But don't be, usually you can get away with about two reductions (done on good tape machines or VCRs) before sound quality begins to get to muddied. At least that is what I have found.
Peace, Jim