The problem of any series with high downforce cars is that racing turns stale. It happens in F1 and DTM, thus they try to implement rules and technologies to create artificial passing. Super GT has another advantage even with its high DF. Having the GT300 on track parallel to the GT500 creates traffic, due to this SuperGT will always seem more exciting since the quick cars are forced to compromise their line. They can't always take the ideal race line. The race length also plays in favor of SGT.
Another example how multi class adds excitement is WEC, I doubt it would be anywhere near as exciting if all you'd see on track were many Toyotas chasing Audis. The slower LMP2 and GTE create many exciting situations.