Re: "It doesn't have to be."
Especially on a bike - you should take air from the highest pressure source available, which may be at the headlight bucket, alongside the fairing, etc.
Re: "Dumping into a huge plenum is as effective as trying to calculate a resistance free path"
Right - after the proper inlet size, you can use any diameter and shape you need (round, oval) to get good flow as long as it ends and expands into a big box. The turbo compressor inlet is not tapered up to the box X-section, but intrudes into the interior and ends in a stack and bell for best efficiency.