Passengers won't notice the descent in fpm (aircraft that is). You could do 1,500; 2,000; 4,000 fpm - most won't notice. There are, of course, extremes to this, but I'll work around the 2,000 to 3,000 mark.
What they will notice is the deck angle, or nose down attitude of the aircraft. Anything more than about 7 degrees nose down starts to get a little uncomfortable for some people, not to mention it becomes harder for the flight attendants to walk up and down the aisle.
Furthermore, it depends on whether the aircraft pressurization system can keep up. For some airplanes I've flown, a 2,000 fpm descent meant catching the cabin around 8,000 feet no matter what I did to the computer. On others you could go up to 3,500 fpm without having to worry about it. I don't usually let the cabin get above 700 fpm as I've found any higher generates one or two comments.