It's well worth your time to scrape that off and put a bit of Arctic Silver 5 on it though, even though it is expensive. Just lets the cpu run a wee bit cooler.
Ach! No no no!
The thermal pads are damn good. (At least the ones that Intel uses. I haven't messed with AMD enough to know if theirs are as good.) It's very difficult to beat them. AS5, once it sets, might give you a 1C difference, and probably not even that much.
AS5 is great stuff, but it's not worth it if you've already got the pad. I only use AS5 when I've pulled the system apart or am not using the stock HSF, so I need to use a new TIM anyway.
Stock hsf units can have large iregularities in the base. For those who are not used to applying a TIM, the stock pad is much better.
Before I use AS5, I lap the base. Since lapping doesn't do much for the A64s, I dont do much of it lately. With a prescott, it is a much better idea, can lower temps by as much as 5c.