CPUs so rarely die or are truly defective that the warranty you are voiding is almost meaningless.
Honestly, I ussually lap to 2000grit and get a nice mirror finish, because it's easier to sell later.
One thing to note is that if you lap both the CPU and heatsink (and the heatsink surface exposed is copper), then using something like LiquidMetal TIM
will lower temps even farther. It works about the same as other high-end TIMs on non-lapped/non-copper surfaces, but after a good lapping it will beat most other TIMs.