My roadbike is faster by about 5kmh than my mtb on slicks but, its not THAT much faster. Don't expect miracles from a roadie. Its the engine that counts. I can hold 30-32kmh all day on the flat with hybrid/mtb/tourer. With the roadie its more like 36-37 for the same perceived effort. Yes, it does have a better top end. The mtb tops out at 46 or so at which point the 48t big ring and the wind resistance is like a wall. The roadie goes faster than I really want to go, but for the same effort (in the drops) it will hold 50kmh+.

Focus not on your gear, focus on you. Speed will come whatever you ride.