First of all Sangakkara is a Great batsman and with Kallis, Dravid, Sachin, Ponting one of the few great remaining batsman of the era. He is not ahead of any of those batsmen listed though and so on that basis alone, and without even leaving the era, cannot make it into any all time 1st team as a batsman.
As a keeper, he just didnt play enough of his career behind the stumps and when he did he keeping wasn't quite up to par with Gillys. Also if we ignore that his average falls when he keeps and we use use his overall average, then Kumar is still behind someone like a Clyde Walcott, who from all reports was a better batsman, especially like Sanga, at home.
On a slightly different point, if we do look for a pure keeper, kindly dont refer to Boucher, who was quite ordinary stading back, and even quite worse up to the wicket. If you want truely great pure keepers take a look at Andrew, Knott, Healy, Oldfield, Hendricks, Tallon, Evans, Taylor, Ames ect, and specifically to pace Dujon, Murray and Marsh.