Have you profiled your execution, and found them to be a source of slow down?
Consider their usage. Are they mostly for error messages outside the normal flow of your code?
As far as reserving space…
Some implementations probably reserve a small buffer before any allocation takes place for the stringstream. Many implementations of std::string do this.
Another option might be (untested!)
std::string str;
str.reserve(50);
std::stringstream sstr(str);
You might find some more ideas in this gamedev thread.
edit:
Mucking around with the stringstream’s rdbuf might also be a solution. This approach is probably Very Easy To Get Wrong though, so please be sure it’s absolutely necessary. Definitely not elegant or concise.