Not sure what we're talking about. Sirloin tips, beef tips or what" Is it pieces or a roast?
Look in the beef forum and there is a thread by Doug123 on smoking a sirloin tip roast. As far as smoking pieces such as beef tips, I would think they would dry out awful quick so you would have to be very careful with them.
Marinated meat is fine. However, steak is really better cooked hot and fast over charcoal about 500-700 degrees. Smoking tends to dry out very lean cuts of beef. I personally would stick to grilling my steaks. If you need to you can put one of the little grill baskets or something (I use a fajita skillet) to keep the little chunks of slices of meat from falling through the grill grids. Cook steak fast and hot and if you want smoke on it throw some wood chips on your coals or in your smoke box over your burner.
Someone else may have more or better experiences with smoking steaks slowly but this is my .02.