One thing that concerns me is her appetite. She has caused us to keep an impeccably clean kitchen because anything that's edible is fair game for her - she's like a dog that way - except that she'll dive into the garbage disposal - you walk into the kitchen and see a long skinny tail sticking straight up from the sink. LOL!
If I knew the answer to THAT question, I would have Cat of the Year all the time . . . ;-)
The show quality cat more nearly meets the Breed Standard that is published by the CFA: CFA Siamese Standard
The pet quality cat is one that the breeder would not want to breed from - it usually has some significant deviation from the standard and so would not improve the breed. I would say that the most common things you will see are a steep "break" or stop in the profile at the eyes (a Siamese ought to have as near a smooth line from between the ears to the tip of the nose as possible), a short or thick or kinked tail, undersized ears, or big feet. Muddy or uneven coloring and pale washed out eye color are two other things that will send a cat to the vet to be neutered or spayed.
So long as your kitty is very active, don't worry about her eating. If she's bounding around the house like a lunatic, she's burning all those calories as fast as she can consume them. If her eyes are clear and bright, her coat is shiny and not "staring" - rough and uneven with spiky patches, and she remains active and has good muscle tone, don't worry. That's just her body type, and if you are used to your standard Domestic Shorthair barn cat that probably has a significant amount of heavyweight longhair blood, she's going to look skinny to you. If she seems to be LOSING weight, loses the gloss on her coat, or anything like that, ask the vet to check her out. You also might consider that an active Siamese may need a premium food to get enough quality calories. I now feed mine Pinnacle, which is a sort of kitty natural health food diet, but when I was showing they all ate Science Diet and Iams dry food, supplemented with thawed frozen horsemeat and liquid vitamins.