An interesting thing about Indian cooking (or any that uses spice mixes really), is the unpredictability of flavour based on variation of ingredients in prepared mixes.  How do we overcome this and is it necessary to even try?

Case in point, 8 Spice Mix found here:- http://www.curry-recipes.co.uk/curry/index.php/topic,5958.msg58893.html#msg58893

3 of the 8 listed components are in fact mixes.  Mild Madras, Garam Masala and Tandoori.

A quick search for recipes to these 3 mixes readily shows that these are not necessarily the same from place to place, brand to brand etc. In fact they can be considerably different.  It also shows that all three of these blends contain most, if not all, of the other 5 single spice ingredients used in the "8 Spice Mix", also in varying amounts. This gives the cook a very poor control over the actual amounts of the "individual spices" actually going into the dish.

Obviously the best thing to do is to find the most convenient source of your spices and mixes, then adjust your recipes to suite the individual taste.  Does it really make that much difference?  I say it does and a simple "Curried Sausages and Rice" made with a good curry powder Vs one made with an ordinary one will show this out.