No, not at all. First of all, don't assume that someone knows what they're doing just by choosing C or C++ over Java. There are plenty of dumb C/C++ programmers out there, and a well-written Java program is always going to outperform a poorly written C/C++ one.
Secondly, remember that Java programs may actually be faster than C/C++ programs. Programs written in C/C++ require more time and knowledge to performance tune. Writing something in Java (or other high-level language) allows the author to spend more time focusing on the big picture issues rather than having to deal with a lot of lower-level issues.
I'm not against Java and I'll even admit that theoretically I could imagine a situation where a Java program ended up being faster, but in reality, that never happens.
In reality, we always end up in situations like utorrent vs. azureus (for those that don't know, utorrent is written in c++ and pretty much better than azureus in every way). In fact, I can't really think of one instance where a piece of java software is better than an equivalent written in c or c++ (outside of developer tools, because those aren't really directly comparable anyway)
The Azureus/Vuze DHT is a lot nicer than the Mainline DHT (which uTorrent supports), it's just not documented, there are no other implementations, and this statement probably does not apply to code quality.
No. C++ may permit more extensive performance tuning, but the same level of tuning shouldn't take any longer in C++ than in Java. And really I'd say C++ as a language is at least as high-level as Java (especially considering templates), just more of the libraries you'll want to build on are shipped separately.