The engine doesn't hit it's peak HP until somewhere around 7K and the HP curve, as I recall, is pretty steep as it's getting to the peak.
At 65 mph in 5th the engine is "loafing" at 4-5K
This means that when you add throttle at 5 K the power comes on gradually until you hit the steep part of the curve. To me it feels like a turbo charged engine getting into it's boost range.
As Jonny says, go down to 4th or even 3rd then twist, you'll hit rocket mode much sooner.