Effective yes. The whole point of this is to keep heat in the turbine. The hotter the air the more heat energy that you are creating to push against the exhaust wheel.
I'm assuming a bonnet is the hood of the car, if so, it does help with this a little by confining the heat to the turbo condom. Problem is, it will still escape from the bottom where there is less coverage, and being that heat rises, will still keep your engine bay a little warm.
The only real concern would be in a high HP high boost application where you would need to worry about exhaust temps. Since you will be dissipating less heat your headers/ex mani will be much warmer than before, and thus you will notice an jump in temperature on the exhaust side.