Tri bars are definitely the fastest that I’ve ever descended by quite a bit. Neither were in race conditions, once was on a mtb with road tires and aero like bars in upstate ny, and once on a tandem with aero bars on the US route that STP takes once crossing into Oregon.

Both times I hit 55mph, and the bike was rock solid riding at that speed on the aero bars.

The fastest descents I’ve done without tri bars has been in the 45 mph range, but I’m sure faster is possible with the right hills.