Don't waste money on the Thermaltake ducting mod!! - Really. I've got one in the spares box! It does zilch except change the pitch of the fan noise. As for turbulence and velocity it sounds like marketing blurb from Thermaltake, although it should, in theory, lessen the dead spot. Don't bother.
Fan adapter and 80mm fan are a good idea, should work fine but . . . .
Take Staz's advice get your case temp down first - it's too high. Intake and exhaust fans required get air moving through your case front to back. Aim for a couple or three degrees above ambient room temp at max!
Secondly if you want to get a good temp reduction from a ducting mod. Cut a hole in the side of your case directly in front of the cpu heatsink/fan assembly (measure it accurately) and fit an 80mm fan (low cfm will do) and blow cool room air directly on to the
hsf. You should get 4 or 5 degree reduction.
Add a cardboard/display board duct from fan to close to
HSF - dead easy, and take another 1C off. Nothing complicated.
I've done it. It works