I think you can just pull that entire horn piece out, is it actually necessary to grind it down?
Quoting keydiver:
The EVO8/9 MAS is only rated at .526 psi pressure drop at 240 liters/second, in case you were curious. More good reasons to choose a #482 3G or EVO MAS over a 2G MAS.
Also, just to repost since it comes up fairly often, the #501 MAS is NOT as good as the #482 or EVO #399 MAS. It is only rated by Mitsu for 210 liters/second, and has ~.78 psi pressure drop because of the larger airhorn blocking the inlet.
I'm not sure if with the horn removed that MAF will respond like an EVO8/9 or #482 but I guess it might, so I'd say give it a go. If it doesn't work using the MAF correction tables I sent you, I'd take that as an indication that it doesn't flow as well even with the horn removed which will mean you'll either have to tune the correction table or get an actual EVO/#482 MAF.