The basic idea behind "overload" and "overspeed" training i.e. swinging a heavier and/or lighter club/implement can be quite complicated. Having said that, there has been research that suggests to only swing a club that is approximately 12% lighter/heavier than your normal golf club. If a club is much heavier than this you will start to recruit different and additional muscles in order to swing the implement. This will also change your pattern of swing and alter your motor pattern. Anything much lighter than normal won't gain you any return on investment, so to speak.
In addition, through lots of research I have learned that getting bigger, faster, stronger will do very little to increase clubhead speed. It is more a function of coordination or pattern sequencing if you will. Hope this helps.