Is this upgrade a thing? I seem to remember Toad saying it was, but haven't been able to find any reference to it or how to equip it. Also, any downsides to getting it?

Yes it is a thing, You can get it right at the start, you have to have the pod upgrade to the "?" and at 10 000 pts you press "B" see my video at 30 seconds : , after that you can just hold the fire button to fire.

AWESOME. Thanks for the quick/complete response 🙂

So pressing B does not do it for me... BUT if i get a red upgrade thinking and move the pod from '?' to speed when i have exactly 10,000 points, thats when it gives me the auto fire (and in your video, you do the same).

Sorry it might be the "A" button (been a while), it might look like it doesnt works but it does, remember you usually upgrade with the "select" button so if you're at the "?" upgrade and at 10000 pts and you press "A" you will see your upgrade reset to the first one, then try to press and hold the fire button it should fire without you having to spam the fire button. hope it is clear lol it's hard to explain and english not my first language 🙂