Welcome to the forum :flower:
Glad you've decided to jump in. Judging by your intelligent questions, I suspect you'll be a welcome addition around here. Please don't hesitate to help us out around here :).
Short answer is all my drives report as SATA I devices. It's helpful to know that for Blu-ray, 1x=4.5 MB/s. And SATA I should be more than enough to handle the 54 MB/s required at 12x. However, it has not proved to be quite that simple. If you could give us the exact model of your burner it may be helpful in getting you on the road to better burning. Perhaps run a sequential hard drive test with something like CDM to be sure your hard drive is not a bottleneck? It may also be helpful to know what burning software you are using? You may wish to try ImgBurn if you are not already using it.
Please download the trial version of OptiDriveControl version 1.50 and perform a read speed, access time, and burst rate test on a burned BD-R? Just start the program, put a check in those boxes, and click start. Then click the "save screenshot"(top right) icon and save it so we can get some more info. Use "manage attachments" button to select and upload the .png screenshot.
As Kerry mentioned, burning media too much above it's rated speed is usually not prudent for important burns. As you are using a 6x burner, you will not likely be pushing your media too hard, provided you do use the good stuff.
Optical Quantum=poor grade BD-R media in recent times. It can be very tough to keep up with who's got the good stuff from month to month. Many brands have been changing the sourcing of their BD-R's from different manufacturers rather quickly lately making it impossible to know what to expect from them. Optical quantum has gone from Ritek to MBI to CMC and usually buying the lower grades from those manufacturers.
In my experience the only brands that can currently be trusted to supply you with A grade BD-R's are Panasonic, Falcon/FTI, and Verbatim. And we need to continually keep an eye on them to keep them honest :). Do let us know what burner you are using. Matching media to burner can help too.