If you know, that amps are not allowed, why do you ask then.
The laws in Germany about that issue is clear; SDR -> SHORT RANGE devices have their allowed output as well as specified duty cycles.
Additionally your imaginations are too simple. Just increasing the TX power doesn't help to reach a certain range; it matters from antennas and their properties, topology of the landscape and other issues.
I clearly tell you to forget about using illegal amps for short range devices - on all bands. Just hooking up an amp to a transmitter without knowing what you actually put on air is highly unresponsible, since those amps come all without any harmonic suppression; so next to your wanted signal, you emit plenty of unwanted harmonics, which can interfere with sensitive radio signals like police, SAR - and about that the regulation authorities don't know any fun!