I've taught Physics for well over a decade, initially in state schools but mostly in independents. About 8 years ago, I made the decision to take my then Department over from GCSE to IGCSE and we didn't regret it. At the time, GCSEs were going through a particularly far-reaching reform which we didn't think was going to serve the best interests of any of our pupils. Lots of other independent schools made the same shift at the time, and more have done since. For a few years, it was a source of much frustration that IGCSEs were deliberately not approved for teaching in state schools, for rather minor technical reasons rather than anything significant.
There are similarities and differences between GCSEs and IGCSEs, and I could type at length on that topic without hesitation, repetition or deviation, but I suspect it would not be voted the forum's most fascinating post. Perhaps I can limit myself to saying that I still believe that IGCSE Physics is a better preparation for Physics A-level (and, IME, teachers of other subjects tend to say likewise), but it's a matter of degree. Thousands of pupils every year go on from GCSEs to do very well at A-level, just as a fair few go on from IGCSEs to struggle.
You're quite right that boosting pupils' results is always high in teachers' minds, both in state and independent schools: what sort of professionals would we be if it weren't? However, it's nothing to do with driving up fees - the bursars and heads I know are desperately trying to stop fees going up further - after all, who wants to price themselves out of their market? - but often feel like they're whistling (or worse) into the wind. But that's a different topic...
Mark