I see more problems than benefits here. Speeding enforcement suddenly becomes a whole lot more difficult; fixed cameras especially. Don't talk about buses, trucks, learner bikers having lower speed limits, because generally these are not enforced anyway. In terms of benefits, I don't see too many. 120kph isn't that much faster than 100kph; many of us here would do 110-120kph anyway along motorways without raising an eyelid. Many sportbikers here would not bat an eyelid at 180kph along a motorway... with a choice between +60kph and +80kph there's not much difference.
Your main argument, apart from the unwritten obvious reason of the personal pleasure of riding faster than everybody else, seems to be congestion on the roads. Having a faster bike limit does diddly shit for that; only a maniac would be lane splitting at 120kph in 100kph traffic (or 110 kph traffic most often, at least in Auckland). Or lane splitting at 120kph in 30kph traffic due to the aforementioned congestion.
Oh and by the way, you mentioned bikes when ridden properly can stop quicker than cars; haven't we had this argument many times before? No, bikes cannot brake better than cars -- the very best sportsbikes with a very experienced rider can by a small margin, but the rest of us, we're worse than any n00b in an old Jap import planting his foot on the brake pedal.
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