The PSU is a custom size; no choice but to try swapping the fan and cut the inside grill if you want it to be quieter. Don't try to squeeze in a 92mm fan; too much hassle, and the airflow improvement from 80 to 92mm is not very significant -- at least at low rpm / noise level.
cutting the two grills for the 60mm fans out and soft mounting the fans
-- good idea. There are better 60mm fans, but I can't point a specific one out. Maybe the SilenX. These fans are probably among the most critical parts of your cooling system, so I'd pay a lot of attn.
The other serious problem in this case appears to be intake vents -- there are only 2, and they seem to have very restrictive grills. My guess is that those high fan speeds (and top vent for the CPU intake?) were absolutely necessary for you to achieve those temps.
Think of the airflow paths from the intakes to the hot components and exhaust ports. The less restricted these paths are, the slower you can run your fans while still keeping your components cool enough. Even though a bigger vent means more noise can get out, the reduction in noise at the sources more than makes up the difference.
I second sthayashi's comment on cheap voltage controllers -- see the Fans & Control section on the main site for reviews of some decent ones.
Don't be afraid to run the CPU temp up to 60C at load. They can all take it; lots of folks here run CPUs hot & never report a failure due to that. I've burned a couple AMDs but only to catastrophic failure -- HS not engaged properly. The only thing you really have to watch out for is the PSU fan ramping up because it's thermally controlled (probably) and the PSU intake is so close to the CPU.
Welcome to SPCR, btw.