Bernhard, It depends on the circuit, however I've seen an RS125 Honda ride around a whole field of 600cc sports bikes on Winton Motor Raceway. Proves what ?
I suggest 'it is horses for courses'. A TZ250 is a great motorcycle when raced off a grid full of 250 and 350cc Yamaha, Honda and Suzuki two strokes. These days in Australian road racing, every grid is a mixed one with bikes of extremely different technologies. We get full grids, but the racing is rubbish. The guys might get a few laps and enjoy themselves, however the spectators might as well stop beside a freeway and watch the traffic for excitement. And it is the spectators who should be paying our bills, not rider entry fees.