Pedal assist makes a huge difference. If I pedal hard when accelerating the engine, I can leave most cars on a takeoff from a redlight.
IMO gearing matters even more than HP for takeoff.
With, engine only, it takes me a short while to get to 30mph. My gearing is high, so engine by itself isn't that quick on a start, but once I get to 20mph, it pulls REAL hard up to 35-40. For that reason, I pedal a LOT at lower speeds, and save the engine for anything over 15 mph. I'm quick in the middle speeds, and FAST at high speeds.
A 6.5HP trike I built for a friend, only topped 30-35mph, but with the lower gears, it would take my bike on acceleration EVERY time.
I'm pretty sure many of you smaller engine guys could easily beat me on a standstill with your lower gears and higher RPM.