Is the curry powder at supermarkets. Much diff than from a Indian store in taste?

just try it and see and then you'll know

It is possible.

That goes for anything, it's called brand capitalism. Supermarket-bought curry powder shouldn't be any different from indian store brands, but Shan is always the go-to brand for your needs and worries, you should be able to find it. It's the mainstay brand in most if not all Indian households. 

In the words of Shakespeare, "Hyperion to a satyr". In other words, no comparison. Actually, if you want your food to taste authentically Indian, you don't use curry powder. You make a sauce using the individual spices to enhance the taste of that particular dish.