I must admit that I have real trouble with the 'experts' that they trot out. Most don't seem to have ever flown anything bigger than a billy cart.
Looking back at the Flightaware data that was posted earlier, and doing some conversions to numbers that are actually used in the coughpit makes for interesting reading. I can't correct for wind, because I don't know what it was, but making a bit of a stab at it....
The aircraft descent seems to be at a pretty much constant mach number (somewhere around .8), until around the 25,000 foot level, when the mach starts to decrease, and the IAS sits at a reasonably constant figure of about 340-350 knots. As I understand it, the A320 max speed is .82 mach and 350 KIAS. So basically, this descent looks to be pretty much what you would expect if you flew current cruise mach number into max IAS. There's a term 'crossover altitude' which is pertinent here. That's the point at which climb/descent and aircraft transition from flying IAS to mach, and vice versa.
If you initiate a descent, with the autopilot engaged, at a constant mach number, the IAS will slowly increase as you descend. Eventually your desired IAS will reach the current mach, and the aircraft switches to flying IAS. If you were to wind the speed dial to the barber's pole (bricks), the aircraft would descend at max mach (minus a buffer, but not material) into max IAS.
So, what would the switch sequence of an emergency descent look like? It starts with putting your mask on, then there is a PA (which was mandated after Helios...serves no purpose), then seat belts, and then you get the descent happening...
1. Altitude dial...turn and pull. It's just flicked down to a low altitude...refined later.
2. Heading turn and pull. Get yourself away from the current track (just in case there is someone under you). For reasons of ergonomics, most people turn to the left, because it's the same dial motion as used on the altitude selection.
3. IAS dial. Pull. That locks in the current mach number.
4. The altitude selection should have commanded the engines to idle, but make sure now.
5. Speed brakes.
6. Speed select the max appropriate.
So where does that leave us? An emergency descent, with full speed brake and very high speed, should give a rate of descent that is appreciably higher that seen here. I'd expect something like 6,000 fpm. But, the RoD does seem to fit pretty well with partial speed brake and the high speed. It almost looks like they did items 1-4, and then stopped.
Pure conjecture, but I don't see flight controls....