IDS should have no effect on 5th gear sound (whine) because it is a product of... 5th gear. I'm sure it's been explained before, but the 5th gear set are straight cut gears, as is 1st gear. If you stay in 1st gear, jocky the throttle a bit and listen carefully, you can hear 1st gear too. At least I can. Most riders don't notice or complain about 1st. because you aren't in it very long (usually) and the accelerating engine sound is a more dominating sound at this point. Anyway, all the rest of the gear sets are helical cut and therefore much quieter.
IDS only retro-fits back to '07 because of the belt width.
As the engine has gotten bigger (again in '07 for OE and even more for CVO) the power pulses become more noticable, not only to the rider, but the driveline as well. These long 'power-on, power-off' cycles, especially at lower rpm levels, create severe back-and-forth forces on driveline components... primary drive, transmission & final drive all have slop. Put your bike in gear with the rear wheel off the ground and move the wheel back and forth. You will feel (and hear) driveline slop. IDS (Rear Wheel Compensation) absorbs much of the 'slop-pulses' that riders were beginning to complain about.
IDS is not free to '07 owners and it's not free to '08 owners either. The increased cost of the new IDS was included in the price increase of the bike for '08.
I have ridden same model bikes with and without IDS, and some bikes before and after the installation, and although I was not particularly bothered by driveline chatter, my '07 is getting IDS, and I will be installing it.
If you go to Biketober-Fest in Daytona next week, check out the H-D demo rides where you can ride an '08 and experience the difference yourself first hand.