Dream747,
When you say SFC, I'm not sure what you're referring to, and what units you are using. If you mean TSFC, which I described in my previous post, then yes, TSFC varies with altitude and mach. But if you look at graphs of such (often kept secret), you would see indeed that the lowest (best) values are at the lower altitudes, other factors constant. I can't really explain why, as I don't know. Perhaps because the air is denser, so there is less energy required to compress it up to the optimal value for the conditions.
As an aside, the "efficiency" of a jet engine is not defined as it's TSFC, but on some ratio of pressures within the engine, if I recall correctly. So don't use TSFC and efficiency interchangeably.
But don't confuse TSFC with the aircraft's performance. While indeed the engine generally puts out less thrust with the same amount of fuel as the altitude increases, there is less drag on the aircraft for the same true airspeed as altitude increases (for normal flights), and hence the SR (specific range, nautical miles per lb of fuel burned) will usually be higher at higher altitudes for the jet aircraft, up to a limit. And this is why I think it is said that jet aircraft are more "efficient" at higher altitudes.