To actually answer your question about which class shaving the grill would put you in, I believe you'd be SM/SMF. I'm basing this on how I'm reading point O from 16.1 ALLOWED MODIFICATIONS
"O. Any minor modification, intended to allow or facilitate any allowed modification, is permitted as long as it does not provide any intrinsic performance benefit in and of itself, does not provide a weight reduction of more than 1.0 lb., and is not explicitly prohibited elsewhere within these rules. This rule is intended to allow minor notching, bending, clearancing, grinding; the drilling of holes; affixing, relocating, or strengthening of brackets; removal of small parts, and similar operations performed in order to facilitate the installation of allowed parts or modifications. Minor strengthening, without relocation, of original chassis/suspension pickup points is allowed. Examples include welding washers restricting control arm mounting."
Though... on second thought... that last bit about "This rule is intended to allow..." really makes it sound like they just want you to be able to install things and make it work. That's not what you're doing with the grill shaving. You're doing it for aesthetics, and the only aesthetic things they've been allowing have been OEM produced upgrades or commercially available aftermarket parts. Honestly you're probably best off just leaving it alone.