Looking to order all Kemet caps for both PSU and amp/audio boards, since the specs are better than Nichicon "audio grade" on Mouser and they cover the range of what I need. The performance specs I'm most looking at are ripple current and hours ratings, along with making sure all are 105C+.
Does that sound like a reasonable route, to go on specs alone? I'm not torn either way on audio grade vs. caps with better specs, so it's not a trick question.
Look at what the caps actually do. For filter / decoupling, your approach is sensible enough. If physical size permits upping the voltage rating, do that. (If that gets you to more than three times the regularly sustained voltage, you can think about increasing capacitance as well.) QC wasn't the same 40+ years ago, so parts back then tended to have substantially bigger margins, often sustaining voltages of multiple times their nominal rating in order to minimize the chance of getting a bad apple. These days the statistical distribution would be much narrower and the average much lower. That's a good part of why they have become a fair bit smaller.
There's a few extra
gotchas. For one, ESR tends to be inversely correlated to leakage current, and leakage current relates to life expectancy in a lightly-loaded scenario. So while extra low ESR parts will stand up much better to abuse inside a SMPS and e.g. last 5-7 years where others might only make it 2, in a less rough environment they might start to fail after little over 10 years while a quality standard part would last 25+.
Leakage also is generally undesireable in applications such as coupling capacitors. Now parts quality has substantially improved over the decades to the point where even a moderately low ESR series like Panasonic FC is quite suitable as an all-round audio capacitor. However, when the original is like a 4.7 µF, a closer look is probably warranted. (If you ever come across an ultra low leakage type, generally orange and most often found near the stereo decoder in a tuner, these very rarely fail. Which is all the better, last time I checked there wasn't more than one or two series of such parts left, so it's a pretty niche thing. These can sustain seeing no substantial DC voltage for decades on end.)
There's also a few tricky cases. Coupling capacitors used to drive low(er)-impedance loads need moderately low ESR, and what remains should be sufficiently linear as well. Audio-grade parts may well be worth it in such an application. And then there are those larger capacitors (often 100-220 µF, sometimes more) at the foot point of feedback networks with the nigh-impossible task of being lowish ESR, very linear and lasting for decades at mere millivolts of DC while often being rated as low as 6.3 V. (The thinner the oxide layer, the easier it is to poke holes in it, and it tends to degrade if kept discharged.)