I'd take a closer look at your overall system and end goals about what you're looking to achieve.
GPU in one x16 and Highpoint 7101 in another x16, but what occupies the other PCIe slots?
What is in your four SATA drive bays?
What is in your optical bay(s)?
What read/write speed do you NEED to achieve?
What total data capacity do you NEED?
How much money are you willing (or wanting) to spend (or "invest") in a machine 7+ years old?
Could the machine be obsolete by the release of the next macOS version?
The numbers IN THEORY and in benchmarks are often different in real-world testing and experience. If you're constantly needing 5,000+ MB/s speeds for system and media/storage, this aging Mac Pro system is not for you. Cut your losses: you've clearly outgrown the machine and its time to move onto another workstation. The limits and bottleneck with PCIe 2.0 are real and impacting your needs.
If you're just trying to eek out as much performance out of the tower as possible, set yourself a budget and go from there.
Yes, SSDs can be repurposed and moved to another machine, but do not assume they can easily (or cheaply) be moved to another Mac in the future (especially true with NVMe). Moving parts to a Windows workstation is a different story entirely.
[doublepost=1548775998][/doublepost]Just as an FYI to anyone looking to get a PX1 adapter: it has been marked "END OF LIFE" by Angelbird. I'm sure retailers will continue to sell their existing stock, but may become more difficult to find once existing stock is depleted.