Welcome to SPCR! Nice design artwork, but it IS overkill and may not work that well.
1) mechanical decoupling is the single most effective way to reduce drive noise. Try it by temporarily removing the drives from the cage & resting both on soft foam -- even one on top of the other. You will be amazed. But it may not get rid of the whine.
2) sidesinks by themselves are plenty enough to cool the drives if you have airflow across them anyway, and if they are longer than the drives.
3) the aluminum plates are not that effective at noise reduction without decoupling; a layer of cork or similar material between the plates and the drive will definitely provide more noise reduction, especially of the highs
4) your best bet might be to carve up a block of foam so that each drive is nestled in the foam, with side sinks sticking out beyond the foam. The drive would be completely encased in foam, with all openings for cables well covered up. The foam does two things --
-- decouple the drive from the case
-- absorb freq higher than ~500Hz
Each foam block could occupy half the sampe you have room for; the 2 together would be wedged together for simple friction-fit. (Sounds like you don't move the system anyway, so...)
the side sinks must protrude at least a couple of inches. They're basically "wicking" the heat to the ourside where it can be tranferred into the air. You could expand the amount of exposed heat radiating surface by connecting the exposed sidesinks with a piece of heatsink material
5) the grooved/ridged/finned aluminum sheeting you're thinking of is used widely in car amps. You might be able to find a burnt-out power amp from a car audio dealer for cheap and adapt its heatsinks for your purpose.
This will be a whole lot cheaper to try. If the temps get higher than you're comfortable with, you can always go back... but probably won't be able to because the drive noise wil REALLy kill you after living with its absence for any length of time