Instead of 500g of cream cheese in the Nutella Cheesecake would adding about 460g make a huge differnce?
Nigella's Nutella Cheesecake (from Nigellissima and on the Nigella website) is a chilled cheesecake rather than a baked cheesecake. This makes it slightly easier to change the quantities but you do need to keep the ratio of ingredients the same. Fortunately we have tested this cheesecake for the US version of Nigellissima using 1 pound (450g) cream cheese.
This quantity is still fine for the 23cm/9 inch sprongform pan though we would not like to reduce the filling too much beyond this otherwise the proportion of cheesecake filling to base goes a little out of balance. For 450g cream cheese you need to use 360g Nutella and 50g icing (confectioners') sugar.