I should have been clear on the propellant - the SSME is a hydrogen engine. For a lot of reasons, comparing engines of different propellants by using a single performance number alone doesn't make a lot of sense. But if you want to talk about hydrogen engines, the Russians built the RD-0120, which reportedly had a higher Isp and chamber pressure than the SSME...

http://en.wikipedia.org/wiki/RD-0120

Although I don't believe we ever tested it on our own test stands, so it's impossible to know for sure.

By the way you are conflating vacuum Isp with sea level Isp (what I was using). The Sea level Isp of the SSME is 366 s.

The SSME is also a fuel-rich staged combustion cycle. The point I was making above is that the Russians were able to get an ox-rich staged combustion cycle working reliably. That's the key technology breakthrough that we have never duplicated.