You're observation is correct. The corned beef brisket your store has is a regular brisket flat that has been injected/pumped with a combination of salts and spices. The brisket can be boiled (as in corned beef and cabbage) or it can be smoked and it becomes pastrami.
When I use one for pastrami, I find the basic saltiness of the meat to be too overpowering, so I'll soak mine in cold water for 4-6 hours, changing the water several times to draw out some of the excess salt. Then I'll pat it dry, rub with fresh ground pepper (I toss the spice packet that comes with it) and smoke it to an internal temperature of around 170. You can take it higher (180-190) for easier slicing by hand, but I have a deli slicer and prefer the thinner slices I can get with the lower final temperature.
HTH
