As far as I know, no specific study has been done on feline muscle tone after neutering. However, several studies have shown that:
- Neutering does not affect a cat's interest in exercise or play
- Neutering leads to weight gain (neutered cats seem to eat more)
In this study on feral cats living outdoors, neutering led to some weight gain and overall healthier cats in terms of coat and body proportions. Feral cats that were neutered do not seem to be at a disadvantage in terms of physical ability.
For indoor cats, weight gain has been noted as a common effect of neutering, but no other significant physical changes are commonly cited.
Edit with your added context: Feral cats who were neutered did not seem to have any increased difficulties interacting with other cats or humans. It did make them less aggressive, so maybe less likely to "defend" their territory, but cats are naturally social, there is no reason why a feral cat would need to chase away other cats. Cats who fight less remain healthier and live longer.