Why don't you take an example from Mike Pall's LuaJIT relying on something more computationally expensive like scimark - he has lua version, and also few other benchmarks (from the alioth site).
If you really want fast sprintf( %s, "%d %d" ) - then you might aswell craft something specifically for converting text to decimal numbers.
sprintf( ) is convenience function, not performance.
I'm sure that's one of the larger goals of PyPy, to make idiomatic code that is convenient to write also perform well. They don't have to be mutually exclusive.
If you really want fast sprintf( %s, "%d %d" ) - then you might aswell craft something specifically for converting text to decimal numbers.
sprintf( ) is convenience function, not performance.