I was fortunate, I was able to "assist" in the spay surgery - as in hand over instruments and take some vital signs, lol (I am a human doc, NOT an animal one) I took her home once it was determined her vitals were normal and she was fine - I am not of the belief that cats need to stay overnight post spay anyway. Unless your cat has a significant health issue already identified, most cats do very well with spay surgery, I gave YY her penicilllin for the 3 days, a protective measure that is the norm here - and recommened by any good vet. (My vet holds a prov post in the assn).
As for personality chamges, that's ridiculous!! Not sure where you heard that but I have seen many many many cats spayed and NOT one ever displayed any personality changes. The only change would be if there was malpratice and say a vet adminsitered too much medication or complications that led to some neurological issues but the chances of that are very remote.
I would not worry. Your kitty will be fine! (Tho I admittedly worried the nite before too, lol)
"There are two means of refuge from the miseries of life: music and cats" Albert Schweitzer