Knowing a cat that is fine though declawed is not evidence that it does not cause problems. I also know a cat who was rescued from a shelter already declawed who is perfectly okay.
Consider this: The UK, Germany, France, Italy, Austria, Switzerland, Norway, Sweden, the Netherlands, Ireland, Japan, Denmark, Finland, Slovenia, Portugal, Belgium, Spain, Brazil, Israel, Australia, and New Zealand have all banned elective declawing procedures.
You can tell a cat that has been declawed just from how they walk, they walk in an unnatural way on the backs of their feet, whereas an un-declawed cat walks mostly on the front pads, much more gracefully and using muscles that were meant for it. Their bodies are designed the way they are for a reason, and declawing upsets that. Cats are very resilient and do learn how to deal with such problems. However, there is no reason to ask them to.
Cats can be in severe pain and you will never know it. There have been cases of a cat who had been declawed who had one of them grow back, curled up inside his foot for ten years, the owner never knew it, but it had to have been extremely painful. The same is true in general, many cats develop chronic pain from a declaw and start acting out because they are in pain, but can't very well just quit walking. They hide pain because it makes them weak to other predators when they act sick.
Declawing is psychologically damaging to cats, as well as causing severe physical pain in the short run and chronic problems in the long-term. It does cause behavioral problems, of course not 100% of the time, but it is certainly statistically relevant and correlative to declaw procedures. Some veterinarians have said that declawed cats live in a "constant state of stress" because of the fact that they are missing a very vital body part, one that is their preferred way to defend themselves, and certainly important if they ever need to. People who do TNR often find declawed cats out of proportion to clawed cats, especially considering that many of the clawed cats were born feral and stayed that way, a declawed cat must have been dumped or abandoned because declawed cats CANNOT go outside. A pet is our responsiblity, and it is our responsibility to do what is best for their health, which includes spaying and neutering them and never declawing them.
If you would like a link to a very informative and extensive site, pm me. It has several graphic pictures so I will not post it publicly.
I truly truly believe that anyone who owns and loves a cat would be wholeheartedly against declaw procedures if only they were fully informed. Too many people, vets included, think it is a routine and harmless procedure, which it is not.