I am not in this GB, and this is my personal opinion. But I take interest in how people run group buys and the conditions during and after the buy.
I suggest going with RNG. I don't agree with charging anyone more than they agreed to at the beginning, nor do I think anyone has priority on a case because of a paying earlier or on time, or faster than someone else. It's simply not fair (in my opinion) to prioritize someone's case quality based on payment time.
If there was a timing separation between the "first 8 or 10" people that are being talked about, then I think yes, they deserve to have priority. Like they paid a week or a month ahead to help make the prototype or something. But if they paid in the same invoicing period, then, they should be treated equally.
If I were in this buy, I'd be insulted if the runner asked me to pay more, for a better quality case. It just equates to how much money I have, and it's so late into the buy. Everyone should be treated equally, unless some special condition were in place. The timing of payment doesn't really mean much, if it was 1minute faster... 1 hour faster, doesn't seem fair to me because of global time differences. People are sleeping or maybe they're at school, or out with their family or something.
In any case, I think people just wants the case. Everyone is already going to get a case with some kind of issue, so RNG will assure the issues are random, and no one got special treatment.
Cheers, and good luck.