Pre-applied paste blows. Not only is performance suspect in factory paste but it's usually applied poorly/too thick. Thermal paste is meant to bridge the gap caused by tiny/microscopic imperfections found on even well-milled heatsinks....so thermal paste should be applied VERY thin. If heatsinks were perfectly smooth then thermal conductive material wouldn't be necessary. I've used AS-5 for years and years and continue using it due to comfort/familiarity. There are better options at this point and AS-5 does carry a few issues: it is electrically conductive so you can't get it on anything and it does take time to set. However, one thing I like about AS-5 is over-time it spreads itself out which means if you applied too much it will thin itself out and help to mitigate that error.
That said, I think I'll get Arctic MX-4 and replace the AS-5. The MX-4 does perform better...but my CPU/GPU tend to stay fairly cool regardless.