It depends. I'm working on a project that uses a lot of C & C++ code for convenience and speed, (but especially speed) and I would say, minimize the amount of interface code period.
Interface code is overhead. Extending the API in C to match exactly what you need and then wrapping the new API in XS will probably give you the "best" results. If you're going for speed you need as few cross-language calls as possible.
As an example, Audio::LADSPA wouldn't be able to generate sound in realtime if you needed a method call for each plugin and frame. That's why it takes buffers of floats and loops through them in C. Looping through half a million scalars a second in perl tends to slow your program down, but C will go through half a million floats in no time. That means it's probably better to write some "higher level" code that does the looping for you, so you only need 1 perl call for a batch of X floats.
Another thing I've noticed is that SWIG's OO interface code is horribly slow (especially when getting/setting properties). That's usually only an issue when the API doesn't really match what you're trying to do, but still it's something to be mindful of.
In reply to Re^3: $perl_extension ? require SWIG : require XS; (XS--)