Well, as soon as you open the psu you void the warranty and you are on your own. After that without a fairly decent lab, especially to measure temperatures in and around the psu, it all gets 'experimental' and depends how much risk you are prepared to take.
A 'simple' mod that moves the existing 80mm fan on a psu from the back (AC input side) to the front (DC exit side) preserves the manufacturer's cfm calculations, but allows you to damp the fan mountings and moves the fan 14cm further inside the case.
After that it gets more 'unknown': on the one hand the psu manufacturer designed (may be an excessive assumption for some psu
) the psu to cope with ambient temps up to 40 or 50C, as well as to run at peak loads for extended periods. They probably also engineered some factor to allow for the situation where the psu fan is the ONLY case ventilation (correlated effectively to the ambient temp assumption). Last, they may even have deliberately 'designed' a stamped grill for EMI reasons, and engineered the fan speeds with the stamped grill as a constraint. So IF
this is all true, and your environment is different (say ambient 25C, normal DC load 90W, and you swap a stamped grill for a wire finger guard) then there should be room to downrate the fan speed.
Looking at Mike's latest review of the 400W Nexus,
the fan there is rated (approx as not sure precise model) 41cfm @ 12V, and runs at 7.1V at 90W (DC) load, so extrapolating that 24cfm should be reasonable for the average 90W load, which is a Panaflo L1A at 12V. As per my para above, if you are prepared to take more risk (or can measure all the psu temps, or assume other things the psu designer couldn't know for your environment like you have decent case ventilation) then dropping that L1A to 5V is an option. (or somewhere in between 12 and 5).
But apart from the standard potential electrical/electronic safety hazards working on a psu, airflow/cooling is your main concern. It cant be overlooked though that if you do remove a stamped grill you change the EMI characteristics, if you break it (up to and including setting fire to your house
) you own both parts (and some discussions have formed an opinion that is how your house insurers may react as well).