top downhill speed in B mode?

That "racket" is the engine being turned at faster-than-usual speed by power from gravity, carried through the wheels, etc. You could get the same effect downshifting a non-hybrid on the same downhill. Top speed allowed by programming under any conditions is about 113 mph for regular 3rd-generation Prius, which may be slightly different for the V. I doubt the programming will allow you to over-rev the engine that way.

 

I tested this yesterday at speeds up to 70 mph. When going downhill around 50 shifting to B caused the RPMs to triple to about 3000. Increasing speed actually reduced rpm since engine braking was not needed. Note B mode will maintain engine rpms around 1000 even when not required.

 

Top speed in B is the same as in D. Not just downhill, but also flat and uphill.

Even in B, there is no fixed gear ratio between wheels and engine. The ECUs adjust as necessary if possible, or as available if full braking just isn't possible, and prevent over-revving. That means at 70+ mph on a 7% downgrade, the engine may spin up to 4800-ish rpm, but simply can't provide enough compression braking to keep your speed down. So the RPM just stays there, and the car gradually creeps up to higher speeds for lack of sufficient braking. But at least it creeps slower than in D, and it takes less foot braking to get down the hill safely.

Despite the high RPM and giant vacuum cleaner or screaming banshee noise, stress on the engine when going downhill is actually less than when climbing uphill. Cylinder pressures and shaft torques are less in engine braking mode than in full power gas burning mode.

 

Nearly everyone here who monitors RPM, uses an OBD-II device to do it. I have an old ScanGauge-II, but plenty of new / better / cheaper products are available.

 

I use a $30 P10 OBD2 device. There are several layouts for more or less data. A button on the back toggles data points. Normally I watch the coolant temp.

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