Wait, you're saying that floating point calculations aren't used in actual scientific computing? What are you defining as scientific computing? Does CFD count for you? I've never heard of any CFD code written using only fixed-point data.

Have you seen this site,

http://bruscy.republika.pl/pages/przemek/java_not_really_faster_than_cpp.html

This guy made a few tweaks to the C++ code (and documents them) and finds that C++ is about 2 - 3 times as fast as Java.