I would like to upgrade the fan as I have enough room for a 25mm one.
As already said, it wouldn't seem a great idea: the stock fan is more than capable, and it's quiet, providing it's firmly controlled by BIOS/utilities (like SpeedFan).
Moreover, there are chance you won't face any noticeable improvements on such a CPU, either in cooling prowess or in quietness.
Please take also note that reducing the height clearance might turn out self-defeating, as it might trigger resonances, and spurious vibrations against the top panel.
I am looking at the following fans:
- Noctua NF-P12 PWM
- Noctua NF-F12 PWM
- Nexus 120mm PWM
In case, I would get rid of the Nexus, as it isn't a good fan at all.
In my experience, the best fan option for a Big Shuriken 2 is a Scythe Slipstream PWM, if you can find one: both the Noctuas are neither more quiet, nor really more effective, despite they usually cost two times the Scythe one.