The speed brake does not seem to have the dramatic effect it has on some add-ons but it does work.  I am not in a position to comment on realism.  I use Radar Contact for ATC and manage my descent so that speed is not really an issue down to 12000 feet.  From there you should have enough space to make your levels through 10,000 while keeping your speed no higher than 250 kts.

So speed management is really more a matter of good planning than trying to make up for it by throwing out metal.  Flight level changes are controlled by speed, so you could try that instead of the V/S.

I don't think your V/S rate cares about airspeed; I believe (but am not sure) it is just going to hit your descent rate.

Please someone with more familiarity correct me if I'm wrong.  I do know that I don't have any problems with speed management.  Maybe learning how to handle a slippery weasel like the MD-80 has given me a better handle on it.

When would you be expected to descend 1200 fpm at 150 knots?  At that speed you should be on approach, and not needing to shed altitude like that.