The dongle doesn't increase the power it allows you to keep assisted pedalling after the standard 15kph cut off. AFAIK there is no power increase available for the modern pedal assist ebikes.For Bosch powered bikes you can by a dongle to update the software.
And you can be legal providing you register and insure the bike. Only fly in the ointment DVLA won't have a clue what you are talking about.