If you are close to or exceed the Max Weight then you might notice that the 'coning angle' also increases. This will reduce performance in all modes of flight since the RAF and IF values will change. You might detect when you have got your figures wrong by the fact that when you enter a 60/60 there appears to be a marked increase in vertical bounce which will increase as you roll on the manouvre. Any irregularity in tracking and balancing will cause the onset of this 'vertical bounce' to ocurr at an earlier point - ie at the begining of the manouvre before fully established and or below the max aum. Another good indicator of a cock up in your calcs - next time you fly try flying an approach to fixed and specific data and note the weight. Now increase the weight and try the same approach to the same data. Assuming you have a steady and similar wind etc then you might note the required power value. I have found in the past that the power required during approach is a good warning of what is to follow.