Hi there, sounds like a good little car you have attained there!
Yeah head work is worth it on all performance engines. However it is only really worth it if you have other modifications to compliment it, such as a performance cam, de-restricted air box or cone filter, changing the inlet manifold for a GT version, a larger throttle body etc. People run various combinations on here so im sure you will get pointed in the right direction.
Toothed belts do exactly what you have said - stop belt slip! This is the main point of them as when the standard belts slip it means that the G-lader is loosing 'effective boost'. Toothed belts stop this from happening giving full boost under heavy acceleration.