How common is it for gastrointestinal cancers to cause an elevated cea level? is this even a meaningful test? does it correlate w/a prognostic factor?

Not for screening: A great many smokers have somewhat elevated CEA, and cancer patients with very high CEA will usually have their cancers spotted by ordinary methods. Every one of the serum tumor markers has proved worthless as a screen -- the extra cases they'd pick up are outweighed by the morbidity (not to mention expense) of fishing expeditions searching for most-nonexistent tumors.