I think your clutching at straws thinking/hoping something is wrong with the bore and that is causing the problem. Sounds like you really want an M5.
I'd identify the problem first then see wether or not you can be arsed to fix it before breaking.
To do a plug like that on over run it would have to be fuelling......a fair fucking bit, sure its no just a weeping injector?
Wonder is Stu can tweak a chip to trim over run fuel on that 1 cylinder, proper bodge though.