It is quite likely that changing the height of your opening won't significantly help. If you are lucky, it will. I have seen people use a piece of metal across the top of the opening that effectively lowers it and sometimes helps with smoke licking out. I would bet even money that there is something more basically wrong with your chimney construction. It is more common for chimneys to be built incorrectly than correctly. If I were you I would get a glass door. It might solve your problem. See informative links below.