I can tell you for sure that the motherboard's boot order is totally irrelevant in determining drive device file mappings. udev decides what order disks are discovered in and in what order they are given letters.
So, if you really want to change them - which isn't a big deal, since you should be mounting by LABEL= or UUID= anyway - you need to mess with udev. Here
is a link that will help; ignore the title and actually understand the info in there, and it looks like you've got enough to create the file that will cause udev to give your disks a specific letter. YMMV, as they say. Feel free to come back if you decide to undertake this challenge and run into any snags.