While China has probably been doing better, I know that the catalogue value of my Indian States have has been rising by a steady 7% or so a year over the last ten catalogue years. I know, because I track the prices.
My collection does include some high-priced (for the Indian States, that is) rarities, but it also includes cheaper items. Cochin SG 76, for example,

has gone from £4 in the 2005 Gibbons to £12 in the 2014 catalogue. Cochin SG 94

has gone from £3.25 to £12 over the same time.
At the higher end, Jammu & Kashmir SG 5a

(The 1866 4 Anna ultramarine watercolour on native paper)
has gone from £750 to £1800 over the same period.
I definitely would
not recommend investing in the Indian States, though, unless you have a very good idea of what you're doing.