I know of several people who have had double ended ignition coils pack in. Initially they keep going but start to misfire at high revs and then slowly get worst. If the spark is weak at high rpm then that wouldn't help. One owner ended up with the ignition coil melting. Sorry i don't know what make the coils were.
There's no obvious performance problem. I'd discounted the coil, because it was common to both plugs.