I'm not doubting you. I just find it odd, since -- completely objectively speaking -- (2d6)-7 is one more computational step than 1d6-1d6. (I suppose they're the same, if you count "pick the red die" as a step.)
The big difference is that, for a lot of us, the computational step of d6+d6 is so ingrained and familiar we really don't have to think about it. It's not "Oh, that's a 2, and that's a 3, so I rolled a 5" it's "Oh, I rolled a 5" - in effect.
Unfortunately, then you get that -7, which mucks it up. Subtraction isn't the hardest mechanic, but the moment you start moving into the negatives, then a lot of people struggle.
When you compare this to 1d6 minus 1d6, then you *certainly* have the problem of identifying which die to subtract from the other. Order suddenly becomes important, where it wasn't for 1d6+1d6.
Eventually, when you've used the mechanic enough times, you'll get to the stage where the pattern recognition most of us have for 1d6+1d6 would kick in (assuming you always use the same dice). Still, that'd be a lot of rolling.
Ultimately, I'd almost prefer to build a deck of 36 (or 72) cards. Mark six (or twelve) of them with "0", five with "1", four with "2", five with "-1", four with "-2", etc. And possibly add a "reshuffle" card as well.
However, I've seen this sort of mechanic before - in Chameleon Eclectic's Babylon 5 - and I'm not too impressed by it. It's of a level of clunkiness that approaches the Siege system of Castles and Crusades.
Cheers!