To pick up 400 points in a year, from master level, is crazy.
Fischer's 3 year rating climb

I'll also note that in the ensuing 15 years he added "just" 200 more--his published USCF max being 2824.
The hill gets steep indeed.
Source?
Chessmetrics ratings aren't real ratings, they are estimated ratings based on their own formula.
Who said anything about Chessmetrics? Source:
http://www.chess.com/article/view/the-first-uscf-national-rating-lists
In USCF ratings:
End of 1955 - rated 1726
In June 1956, he met Jack Collins, a chess lover who had an extensive chess library, Fischer dived right into this.
End of 1956 - rated 2231
End of 1957 - rated 2626
Pretty crazy stuff.