There is nothing to worry about if a heate gets up to temperature quickly (mine does too) as long as it doesn't go above the normal range into the hot band. But this 'problem' coupled with your poor heater problem, leads me to think you might have an air lock. Have you changed the coolant recently, or had to top it up? Sometimes when changing coolant you can introduce air locks which can be difficult to remove.
If you haven't changed the coolant for the last two years then I would recommend that you do, and change the thermostat at the same time. It's not an expensive job and well worth doing in readiness for winter, particularly if the coolant has been neglected as you could have silt accumulating in the system. If your cooolant is old and dirty then you need to flush it through with a hosepipe to get rid of any silt before replacing with new coolant.
