It voltage rating might be more safety as AC does have peak valve of 1.414 x the RMS (assuming 60 hz) value that most of us are usually see it listed as. Here our 240 vac is the RMS value with a peak value of 339.36 volts and the is the peak to value that is twice that. Our line voltage is supposed to be 240 here but it actually measures 260; just a way the electric company got to push a little more electricity thru the line so we use more. So 450 vac rating is within a 25% safety factor reason. They actually back in the 70's were calling it 220 when I was learning electronics.
And from what I understand you're right on the starters. It's the run capacitors that the value needs to fairly close as it controls how much current is used; too little motor won't produce the rated hp and too much just wastes it as excess heat.
Boy I haven't used any 450V caps in a while since vacuum tubes went away like the dinos. Sure help make good foot warmers in the radio shack on cold days. I always like the blue glow of the those horizontal output tubes when things were tuned right. But my longest radio contact was a fellow in Queensland on 1 watt. I had fellow operator running 2KW couldn't even make contact.
