I found this site
that suggests the Duratec is quite lightweight. I stand very corrected.
Of course, there's water cooling of the controller, but the radiator will be much smaller than for an engine. Your Polo radiator might be lightweight, but how heavy is it when it's full of coolant?
The loss of a gearbox is not a good one. You're a man of calculations - check out what difference a six-speed gearbox makes to acceleration, even with the extra 45kg. Having 2.5x the wheel torque in first means 2.5x the acceleration (or wheelspin).
Compared to a car engine, I think electric wins. Compared to a bike engine, even a standard one, I think it's slightly heavier for the same power. However, the spread of power over the rev range may make all the difference. I'm proposing a combination of the two - augmenting the power of a bike engine with some electric assistance.
Using a DC motor instead of a brushless one means that you can use a contactor on full throttle instead of a controller. It's both lighter and has no requirement for cooling. The down side is that the power is either on or off, which isn't a problem when you're talking about 20% more power in a straight line. I'm also thinking of attaching it on the engine side of the gearbox, which means you benefit from gearing. I've done a little simulation and I think it's worthwhile - I can give you some raw data if you want to run your own simulations. Also, being smaller motors, they're more popular, which means they're also cheaper. The whole kit comes in at a lower cost than a cheap turbo kit.
I drive a bright orange ADR in sprints and can often be seen spectating at hillclimbs. I'm always up for a chat, especially about electric things.