I am at a loss - any suggestions on the next step?
If you hear that your car is misfiring, I would make sure that you actually
did replace number 8, and not the other side.
These engines with coil-on-plug, are easier to fault find, since you are not
dealing with high tension on top of the coil.
Remove the air filter, and the cover. Then remove the low tension connector to each coil
in turn. Even more degraded running, its not that cylinder. No change, you
have found the candidate.
When you have found the cylinder, do a test where you swap the coil to
another location, but keep the plug. If the fault moves with the coil, well, its
the coil. If it stays, it should be the plug. Also do consider the high tension lead
between the coil and plug, in the same way.
I still think that you have water down one of the plug holes. Either blow it
off by air, or warm the engine up, without the cover on, to let it evaporate.
I've had both. Misfire after a to heavy engine wash, and a faulty coil.