I have the 1999 version of the 21 DV, as well as a EFI Yamaha 200 HP. I also don't have a fuel-meter, so I can't answer all your questions.
For what its worth, my max speed is 38 knots on the GPS, which, according to Google is 47.8 mph. I am running a 13 3/4 x 17 prop if I remember correctly. I believe that was 3/4 tank of fuel and solo, with typical fishing and safety gear.
Again, without a fuel meter, I have to guesstimate my most efficient RPM.. most people say that is right around 75-80% of WOT for a 2 stroke, which puts me around 4125-4400 RPM. I typically cruise at 4200 RPM and get anywhere from 24-27 knots (27.6 - 31.0 mph) depending on sea state and tides.
I haven't been on a 21DV with a 150HP, but I would recommend going with the 200HP. I think the boat is rated at 225HP, so 200HP seems like its in the sweet spot. I know others have said that a 150HP is ok on the 2120, but if you are running an inlet with a following sea against the tide, you really need to get power when you need it.
Again, going by memory here, but I think I average anywhere from 1.75-2.25 nautical miles per gallon on long trips of 40+ nautical miles.
-- Tom