RE: Can we replace stock Dell case fans? XPS 8500
Those Corsair ML fans are 120mm/140mm only, so you can use them only in a front intake position. They also seem to be 4 pin PWM fans. They can run as 3 pin fans however by just not connecting the additional speed pin.
You can certainly change the rear exhaust 92mm fan in the XPS 8500 with a different model than the ML fans, just make sure it is a 3 pin fan, as that fan header is controlled by voltage. It may not change the circulation much however as the fan speed won't change much since that is controlled by the fan header. If you connect a rear fan running at a higher fixed RPM to a 12v molex connector and leave that fan header bare, you will get a error at boot. The way around that would be to install a intake fan up front and connect that to that 3 pin header.
It does help airflow to add an intake fan up front like DanH mentioned. Usually 2-4 degrees drop in CPU temps from what I saw. You won't have to worry about sensors by just connecting that fan to a 12V molex from the PSU with a 3 pin to 12V molex adapter. A large, slow moving air flow fan is best running at 1000-1200 rpm, one that can move a lot of air without a lot of noise.
Doing both would be the best best, a new rear exhaust fan running at a fixed 1100-1200 rpm, and an intake fan up front being controlled by the 3 pin header.
If you are planning on running a GTX 1080 class card on a 4K or 3440 x 1440 display, then things get a little more complicated as the CPU temps will become an issue.
More info here on the fan headers: