Depends - during the winter after a freezing rain when the bike paths are a mess - safety requires that even with studded tires, the commute can be a crawl across frozen slush that can often hide foot-deep water.
During the spring before the bike paths get crowded, I can usually average about 35km/h on a 15km commute. During the summer the paths are crowded and I don't bother pushing things. Average drops to about 23km/h.
But when I feel the 'need for speed' -there's always the winding roads that go over the Mount Royal summit downtown (elevation 750ft more or less). Freshly paved this year - ideal for road or mtbs with slicks. Early in the morning before there's any traffic on the road - its a breeze to hit 90 going downhill.
But I'm thinking that compared to this guy I'm just a kittycat: http://www.flixxy.com/mountain-bike-...eed-record.htm

But you're pretty quick! I regularly check the computers on the bikes that come into the shop and almost all fall below 20km/h - even the super fancy CF road burners