In general, high back pressure will hurt performance, (clogged cat, smashed exh pipe, etc.)
You mention max allowable, but not @ what rpm - @ idle, 2500?
The back pressure will change dramatically @ different rpms (more rpm, more pressure)
Do you have stock exhaust?
13in. water column is just that - a column of water 13" high in a tube - you could blow that easily.
I doubt that that would cause a misfire