Honestly my cat is so skittish. You can flush a toilet, and he'll jump. And he seems perfectly ok. I do recommend getting feliway. It seemed to really help him chill out.
One of our cats is like that too. She also follows me everywhere, she has to be near me all the time. Sometimes I think she's a dog in a cats body! She really freaks out in the car, so she isn't a dog in that respect.

When I took her to the vet earlier in the week for the microchip/rabies shot, she was meowing so much more than the other one. When the vet listened to her heart she had a heart murmur, he said that can happen though. Somehow I think they'll feel better if they can smell/hear the other cat in the plane with them?
I've Never heard of feliway, I will have to check it out. Thanks for the info!