I'm running 36 PSI all around, which is above specification for my 15" wheels (31 PSI),
I'd agree about the fuel economy win. For my commute I reckon the difference is 7-10% (extra-urban, 20 miles over 35 minutes, obsessively following the green/blue eco-nanny on the dash)
But that's probably different to a typical jazz driving environment. For mostly short journeys in town or stop/start traffic, fuel economy will be dominated by engine warmup and traffic conditions, so you wouldn't care about tyre pressure.
IMO it rides a bit louder than specification pressure, but the MK3 jazz is loud anyway. It doesn't make much difference.
Also feel a bit more transmitted vibration into the chassis, but it's not very significant, and as above the jazz isn't a very refined car in the first place.