Amla has a brilliant Test CV, though the recent years have tarnished the glow slightly.
I don't think that should be held as a huge negative against him, Sachin's average did dip from close to 56-57 to under 54 by the end, doesn't mean we put him below Sangakkara and Kallis.
What is impressive about Amla is he's had impressive, series-winning contributions in Australia, England and India (well, 10 balls away), tours where he absolutely dominated from start to finish. Then, he's batted No. 3 in SA, where it is always tough for top-order batsmen. I don't know where I'd put him exactly, but like Pietersen, he's probably a bit higher than people with a similar average as him (Jayawardene, Younis Khan, Clarke).