I made some measurements using my miniVNApro with a simple test fixture fittig the QRP labs lowpass filters.
I've measured all low-pass filters in the shown test fixture. They show a pretty good attenuation. The first harmonic is suppressed more than 32dBc on all filters.
Keep in mind, that with the current U3s relay board, this suppression is reduced due to some design decisions made by QRP-Labs. Check also the resulting transmitter signal here.