XPS 1645 Heat Sink cleaning?

Hey guys, I've been lurking these forums for awhile. I have a 6 month old XPS 1645 (i7 720QM and 4670) I purchased from Dell.com. I mostly fixed throttling issues I had by using BIOS A09 and removing the vent covers on the bottom of the laptop. The laptop now only throttles when running Prime95 and Furmark.

My concern is that the laptop seems to be running hotter (5C) now than a couple months ago while playing games (only comparing to the time since I starting using BIOS A09 because I know A09 can get hotter since it doesn't have the 84C limit). The room temperature is actually lower now then before and the filters are removed, so I would expect lower laptop temps or at least the same while playing games. If the laptop keeps getting a little hotter, I'm worried that it might start throttling again during games. I tried using compressed air on the fan, but not much dust came out.

My questions are
1. Could my heatsink be filled with dust? If yes, can it be cleaned without unscrewing it?
2. If I unscrew the heatsink screws to blow it out with compressed air, do I need to re-apply paste, or can I just screw it back in? (is there any harm in removing the heatsink and not reapplying the paste?)
3. Are there pads AND paste attached to the heatsink? If I were to reapply the paste (hoping I don't have to), do I save the old pads/paste?

 

To clean out the heatsink you don't have to take it off. All you need to do is just take off the base plate and then clean away. You don't even need to take off the base plate, you can simply airdust through the vent slots (though this probably won't get rid of as much dust). Also, make sure that there isn't dust collecting in the mesh on the bottom of the base plate where air enters the system, dust seems to collect there a lot for me.