Are you using a Mach3 post in Visual Mill, or some other post? I'm surprised if it's in a Mach3 post for VM.
I found a post that said in old versions of Mach3, is was to select advanced comp mode, but it's not used anymore. I've never seen it used, so it must have been a very long time ago.
I would assume that if other people are using G40.1, it's probably because they don't know it's there.
Should you change it to G40? Well, if it was originally there for a different purpose, then I'd say no. But, it's a good idea to have a G40 at the start of your code, so if you don't already have one, then go ahead. If you're not going to change it, then I'd remove it. Anything not recognized by Mach3 shouldn't really be there.