I'd buy something other than AS5 .... look at their web site ..... it cautions that it takes over 200 hours and numerous full thermal cycles to fully cure. Benchmark reviews site estimates that it will take up to a year under "normal PC usage" to fully cure AS5. Shin Etsu has the exact same thermal performance w/o the cure time issue.
Whether you get a new cooler will depend on what CPU and how far you want to OC. The 2600k for example has no issue with 4.5 Ghz and the stick cooler.