Unless you utilize software which can make use of the extra cores adding more is unlikely to result in any perceptible speed.
Booting a system is primarily disk bound therefore it is not surprising to hear you did not observe an improvement in boot times. The SATA-II (it is not SATA-III) is unlikely to be an issue with your boot time. OS loading primarily consists of random reads which do not approach the limitations of the SATA-II interface. Therefore moving to SATA-III or NVMe will not appreciably improve boot time (courtesy h9826790):
As you can see the boot times for the three different interfaces are essentially the same (the rightmost bar represents the configuration you have).
What do you use your Mac Pro for? What software are you using?
I often use Adobe CC apps, like Photoshop and InDesign, I create both text and pictures, I write and work with a magazine in my village 5 times a year. I also work with web. I would probably be able to do this with a Windows-computer, but I have never owned one.