my cat, klaus, has recently been diagnosed with the same type of cancer. they found a mass in his intestines and said his spleen was enlarged from the ultrasound. they conducted surgery and removed the tumor from his intestines and also saw that a lymph node was affected and very large, but were unable to remove the lymph node since it was such a vascular area and there was potential for him to lose too much blood. i don't think they removed the spleen. they did a tissue biopsy of the lymph node and it came back as a mast cell tumor.
my vet has told me that chemo doesn't prove to be very effective against this type of cancer but that steroid treatment can stave off advancement very well, maybe even a couple of years. similar to your kitty, klaus has displayed minimal symptoms. he was throwing up frequently for a couple months (which is what brought me to the vet in the first place), and he would have periods of the day when he was lethargic and hiding under my bed. but he always seemed to snap out of it after a couple hours and was back to his energetic, playful self.
i've been scouring the internet for information on intestinal mast cell tumors and there's not a lot of information out there to help me figure out what the next step should be and what the prognosis may be. it seems not very hopeful for cats, but klaus is only 5 and i hope he can fight it off for a few more years at least.
i am also in NC (raleigh). i was wondering where you are and where you took kitty? and also if she is still doing well? thanks!