I cannot remember driving that fast even on the Beltway here in Houston. I think I got up to 70 but that is about it. I do not think the Prius is designed to travel really fast myself so I do not push it.
Oh there wigglers. Short wheelbase car with zero deadband steering and thin tires. Tends to track every little malformation in the road. Requires constant attention to driving. My one big complaint in a Prius. This wiggling issue is slightly exacerbated by running the tires at door placard tire pressure which is really quite low but will exhibit a very nice ride. Pump up the tire pressure in the front tires to 42 psi and backs to 40 psi. It will really help the wiggling and REALLY help your gas mileage. The downside of higher tire pressure is the car will ride a little rougher and hit bumps harder though. Some people notice it some don't.
I see the op is in a gen 1, I have a gen2 and it was loose, beefier mid brace, upper front strut bar and larger rear sway bar made it an excellent vehicle And a gen3, it got a beefier mid brace and a rear sway bar, not as tight as the gen2 but not bad just sayin...