it's not that much of a pain. from start to finish, if you've never taken it apart before, and assuming you have the schematics and know what you're doing, it shouldn't take more than 30-40 minutes. The first time I took my 9300 apart to drop in a new CPU, and it was the first time I had ever taken any computer apart, took me about that much (maybe a little longer) for the disassembly/assembly. Add about 10-15 minutes for removing the thermal pad, cleaning the heatsink and chip, and applying AS5. The second time I did it, it took half that time.
The temp drop you're going to see, as stated above will be around 5 degrees C. Yeah the thermal pads that dell uses are crap, but if you're not worried about your current temps, I wouldn't bother with it, as stated above it will void your warranty if Dell finds out. That being said, if you're planning on doing some work under the hood anyway...dropping in a new GPU, or CPU...definately put the paste on it, it will drop your temps and, given these activities, you'll already have voided your warranty.