I don't think you would have to unmount your mobo to apply thermal paste.....?
The cooler you have is fine, you just have to figure out whether the temps you're getting are normal or not..... Keep in mind, there's a huge temp spike before the thermal grease sets in......
I can give you tips on applying thermal grease.... For example, you actually would want to polish the surface of the heatsink WITH thermal grease before applying any..... Make sure before you do anything, that the suface is completely clean use isopropa-something alcohol or rubbing alcohol..... Spread the paste with you finger through a bag and smooth it out with a flat edge, ex. a playing card or credit card..... One think I personally do is right before apply the heatsink is to put the tiniest (i mean THE tiniest) little dot of thermal grease right in the center of the CPU and let the heatsink smooth it out (to avoid hotspots). And try to apply the heatsink as squarely as possible and try not to rotate or anything so you don't create any hot spots..... (contrary to popular believe, you only rotate the heatsink if you did not smooth out the thermal grease before hand.....) After everything is set up, try to run the CPU as hot as possible (without damaging it) and run it for awhile, then turn it off and let it completely cool so it allows the paste to set in. (but it still take a whole week before you reach your base temps.....)
Edit: Didn't see archs post..... These tips only apply to AS5, not zalmans stuff.......