Hi, I think your PC can pull quite a lot of w so will need to be careful.
On plus side as is water cooled presumably there is little heat in the case for the PSU to be sucking in?
Flicked through some reviews of the PSU and looks like it does have over-heat protection built in so should shut down before cooking itself. Couldn't tell what sort of fan it has in it.
I have an Antec Signature which is similar in that it's straight through 80mm fan design. The stock fan wasn't bad and could get down to ~750rpm using 4 wire PWM control, but wasn't super quiet at that. I believe it can go all the way to ~4000rpm and must be ear splitting!
I've swapped it for an Arctic Cooling F8 PWM fan, on soft "Acousti" mounts, plugged in to the PSU's header so still controlled by it and with the speed wire lengthened and routed out the PSU.
My Q9400 + GTX260 can pull more than 300w AC and while I think your PC may be able to pull more I think won't be too far away.
The fan in my PSU at idle drops below level where rpm is reported but I suspect it to be ~500rpm, under load it will get to 700~800rpm and with extreme benchmarking I've see ~1000rpm, still well below the fans max of >2000rpm so I think the PSU hasn't come anywhere near it's maximum temperature. (then fan speed is based on internal temp of the PSU, affected by load and air intake temps)
I would think you could duplicate what I've done with an F8 or F8 PWM depending with your PSU needs but would strongly advise being able to read the fan speed and checking it's not getting to close to the fans maximum, ie the maximum temp the PSU wishes to be internally.Pictures in this thread.
Your 2600rpm fan should also work as long as PSU works on voltage not PWM control. You can't add the fan CFMs together when fans are in series. As the PSU is a restrictive airflow path 2 fans in series will push more air but probably not double. I wouldn't bother with two fans until proved that needed, but sometimes two slower fans can cool better one fast one and more quietly.
Rear of PSU will be very hot environment for fan so bearing type/quality maybe important. Perhaps not sleeve bearing, AC fans are some sort of FDB but still very cheap so who knows how will last - OK so far!