Willow: Around the other way. I think.
Instrument overhead is normally 554 bytes though your tracker MIGHT only save the 547 or 548 bytes it actually uses. Unfortunately I only made munch.py AFTER WC6, but that works a charm with instruments.
My advice if you don't have something like munch.py: don't use instruments, they have a lot of overhead. Just do it in sample mode instead.
Condolences to those who use .xm, which only has an instrument mode.