I think that one of the worst impacts can be that the increased solubility of petrol cleans the tank of any accumulated crap that is stuck there and this then clogs fuel lines. While it doesn't do too much damage it's pricey to clean all that piping again.

This is what happens when you use biodiesel in too high concentrations with an oldish diesel, too. There was a very high-profile case in UK recently where a supemarket petrol station added 5% biodiesel to the diesel they had been delivered without realising that the stuff they had was already 5% biodiesel. As I recall about 5% of the cars filled with the stuff had an engine failure.