If the tailwind component briefly reached 200 kts, then only about .86 - .88 mach (depending upon temp) would be required to achieve 500 KTAS and a GS of 700 kts. The highest GS I ever saw was 605 kts. And that was at 425 KTAS (about as fast as a typical Westwind is likely to go) The UNS 1 showed the wind right on the tail at 182 kts.
200+ knots jet stream velocities are not so uncommon as to be particularly surprising. Turbulence should be expected whenever there is a large difference in wind velocity or direction over a short distance. Unless it's a particularly lucky day, you'll find it!