Before you start pumping out that kind of power I'd invest in a spectrum analyzer (which are surprisingly affordable today) to ensure that the signal that goes out is clean.

Those mini SDRs when used for transmission will output a ton of harmonics because they are almost by definition not going to have any filtering on their output because that would limit their usability. When used for reception that is not a problem, if you start using them for transmission it's a completely different matter.

Keep in mind that a power stage that responds well to a particular signal absent filtering is going to respond quite well to the main harmonics too!