Overheating after hard drive replacement

Within a few weeks of replacing my iMac's hard drive (following simple instructions here and on youtube) I noticed that the fan was always running fast (and loud). iStat showed that from a cold start, the GPU was up to 89 C and the GPU diode at 101 C within 15 minutes. All cools again slowly if the screen sleeps for long enough.

No ill effects, but very noisy fan in my dead quiet office. CPU fan runs constantly at c. 3300, which I understand to be close to max.

First I used canned air to clear the vents. No sign of dust expelled. No reduction in the above temps or fan speeds.

Second I removed the glass and the screen to access the fans, and cleared these with canned air. Some dust evident, but not a huge amount. No reduction in the above temps or fan speeds.

I was wondering of there was a knack to reassembly that's required to ensure airflow isn't compromised?

Any one else had these issues, or any thoughts on remedy?

D

Worth noting that I also took the whole unit to the local Apple store for a genius bar appt. The guy took it out back, claimed to have cleaned the vents in a huge loud of dust with an air can, said all was OK and that the CPU fan can spin up to 6000 rpm. Given the large amount of BS, I didn't feel too encouraged when he told me that the temps were fine and not to worry.