The theoretical maximum speed for a SATAIII connection is 6GB/s. I don't think 6 megabits per second is going set the world on fire 😉. Notice the use of the word theoretical - you are unlikely to see anything remotely approaching that in a real-world application (SATA SSDs are still much quicker than standard hard drives) - much like the fuel economy figures the manufacturer says you can get out of your car/truck/bike (delete as applicable). PCIe SSD cards are available but I would advise you to check very carefully that your board supports them, I've skimmed through the manual and I think it's highly unlikely but I'm prepared to be proved wrong. They are also MUCH more expensive that a standard SSD so I would probably just add another SSD and keep the change.
Failing that and if you're feeling flush, swap out the mobo, processor and memory and get your added horsepower that way - in addition you could then swap out SSDs for M.2s later if you felt so inclined.