E-bike tuning and speed
When we talk about e-bike tuning, we are not talking about increasing engine power, but rather about lifting the support restriction at 25 km/h. This means that you will be supported by the engine even at higher speeds, but don't expect that you can suddenly ride 50 km/h. Because for this you would need - depending on the bike - a good 1000 W power. And since the e-bike can only support you with an average of 250 W in the long run, this will be quite a sporting endeavour.
Performance and speed calculator on kreuzotter.de
You can use this calculator to get a rough estimate of the maximum speed that can be achieved for a given power, or the power needed to achieve a desired speed.
The rolling resistance is independent of the speed. However, the air resistance increases quadratically and plays the decisive role in addition to the total weight in the plane. For this reason, high speeds of e.g. 50 km/h on level ground can only be reached with several hundred watts. However, a pedelec delivers a maximum of 250 W in the long term. Anything above this must be provided by the cyclist alone, using muscle power. Realistic power values over a period of one hour for average persons are around 150 W.