I've seen bad results and really good results. Depends on skill and patience of the craftsman.
I just pulled the cover and skipped the mod, got to the part of making a thicker gasket using silicone and froze up. Jason Pittenger has a thread going where it looks like he's handled the gasket gap problem in a good way. Been kinda impaired lately so I may not get everything right if I try to tell you.
Cutting the case away from the engine is the best plan, much easier to see you are getting an accurate cut. If I had to, I would use the machine shop at my old job, but they closed the doors for good last month. 75% of a global company, gone.
Aluminum cuts super quick and easy, going slow is key. You can remove, but can't put it back. Maybe draw a line, then sneak up on the line. Could make final trim even with coarse sandpaper if you absolutely had to. Need a CNC, anybody got one they can loan?