yes it's a plenum 🙂
inside it's smooth, so doesn't affect anything, I almost cut it off to clear my 100 Amp alt.
if you cut it off the hole is much smaller, all you need to do is seal the hole, araldite or an epoxy resin should do the trick with the cut off piece suitably cut down.
it might be an idea to do this so the 100Amp alt doesn't need modding dunno..
