Its not very stealthy though... 

Good points...  They will appeal to some out there.

And what performance increase would a better coil give exactly?

I like the idea of them and similar products are available for twin cams as well, some blokes on here have used them for customisation purposes, Stropp comes to mind but they aren't really a performance part, I'll leave it at that I think.

Poor old coil would run pretty hot !