I think I have the same issue, mine aren't running at full speed either and I can't work out why. I hadn't thought it could be the OS but you could be right.
Yeah, I installed Windows 10 on my friend's PC recently, but I didn't think to check at the time. He uses a motherboard with the same controller and the same SSD, just a single one.
What sort of speeds are you getting in AS SSD and what SSDs are you using?
It reads to me like an issue with your Controller, maybe the drivers for it still need to be able for full compatibility with Windows 10?
I was thinking it might have been something like this, I did have a play around with drivers to see if there was anything new.
Google isn't returning much in the way of searches either unfortunately. I've been tempted to RAID the 2 Sandisk Ultra Plus SSDs (as they're new, I haven't used them properly yet) I've got and installing Windows 10 on them to see if I get different results.