Well first of all, we are talking about two wings that could be completely different in strength. For instance, if it was Lopez's forehand vs Safin's backhand, which would come out the better? But if it was Nadal's forehand against Haas's backhand, it would be very different.
It isn't just Paire who's BH is better than his FH. There are at least a dozen other examples off the top of my head that I've already stated.
If we look at the year 2003, which I believe had a field of players who had varied styles and a lot of creativity I can list these players off the top of my head (all must be within the worlds top 100). I think this is a better sample than today's 'consistency' tennis. Better sample because these players were all playing attacking tennis, thus required more sound technique. These are players I think had no doubt better backhands.
Agassi (More consistent wing by far, ABSOLUTELY NO DOUBT that he could change direction more easily)
Safin (I've repeated 1000x)
Schalken (Way more consistent, technically not even close)
Nalbandian (Similar to Safin, did everything better)
Bjorkman (Horrid forehand, massive gulf in ability)
Haas (Flattens the ball better, doesn't need to loop it 15 feet above the net when he's off the boil)
Corretja (one of the most technically sound backhands ever)
Gaudio (Pretty similar reasons to the rest, could produce flatter balls, change of direction with excellent disguise)
Kafelnikov (One of the top 3 double handers of all time for me along with Agassi and Safin)
Martin (leaked far less errors off the backhand, could drill it pretty hard when he wasn't lumbering around)
Gambill (His forehand was all over the place, didn't know whether to hit single handed forehand or double handed)
Kuerten (Maybe the best single hander on slower surfaces)
Stepanek (Not even worth mentioning, his forehand can be Bjorkmanesque)
Mantilla (Masterful slow court stroke)
Ljubicic (Has always been more consistent, forehand is very liable to breaking down and especially on low balls)
Probably more out there that are arguable. I'd also take the likes of Verkerk and Santoro's backhands.