Good lord, if you plan on eventually overclocking, and are going to be using one of the all in one water cooling solutions, take what reviews say about the pre-applied thermal compound with a huge grain of salt. While this was my first time using a water cooling setup, it wasn't my first time around the block in regards to building a PC. I ended up buying the Kraken x60, and my idle and load temps seemed ridiculously high, especially at idle (33C - 37C) when the room was at 20C. Prime 95 would raise them to 72C (on the hottest core). That did not seem right at all considering that's the temp you're supposed to see with this cooler with a good 500mhz - 700mhz OC.
I normally use Arctic Silver 5 on pretty much every heatsink, but I didn't bother this time because most reviews claimed that the compound with the Kraken was "good". Nuh-uh. After wiping that shit off and applying some to the heatsink and then wiping off the excess (enough to simply fill in the small gaps that you can't see) and applying it to the CPU, my temps dropped by 6C and will drop an additional 2-4C after it finally sets.
Also, for those considering a Kraken x60 (I'm assuming the x40 suffers the same problem), you might want to consider simply using the motherboard to control the fans. The buggy-as-shit software doesn't even show CPU temps on mine (it's simply blank), and only the coolant. It also controls fan speeds based off of coolant temps, not CPU temps, which is beyond stupid. Just use something like Speedfan, or if you're using an Asus board, the AI suite for custom fan profiles. It works much better. Although you might want to consider making the temp breakpoints 10C lower than you really want for fan speed, because the Asus software seems to be measuring the temp of the heatsink, not the actual cores (like RealTemp and Core Temp do).