I have a GT with the 1.9" Blackjack tires and see no need to change for speed. I average 21kmh on my smooth track rides with a max size 58t chainring and the only reason I can't go faster is the gearing and bad aero ergonomics. I can get up to 34kmh before gearing runs out, but I think aero is the problem above 28kmh, not the bike. The tires make noise but roll fast at the max pressure rating. 406 tires will allow higher speeds but will be more dangerous on bad roads.