Using a faster version of sv_derived_from?

[Jeremy White]

Hi,

I have an SV created like so:
sv_setref_pv(object, "MyClass", (void*)self);

There are lots of perl/XS calls, and using the function sv_derived_from to 
check that the right type of object was passed is very slow. Are there any 
safe alternatives/work arounds? Oh, the SV is also overloaded:)

Cheers,

jez.