I am pretty sure the Robin doesn't have inverted fuel and oil systems,
Generally true, although I think the factory did produce a few with an inverted flight system just before it went under.
So in the Robin you'll never be able to go beyond the "Sportsman" level. But at that level it's possible to compete with, and even win, from Pitts', Extras and Fireflies (I just proved that last weekend at Connington).
Like I said, it's a good aerobatics training aircraft which will remain interesting for three years or so. After that, you will want to step up to something better. But the same goes for a Decathlon.