Completed the comparison:
1: ProStick Plus without external filter
2: ProStick Plus with added FA filter
3: RTL-SDR BLOG V.3. with RTL-SDR BLOG LNA
The weaker signal at 1090 MHz for the ProStick Plus is not a shortcoming of that dongle, just different gain.
But even with higher gain at 1090 MHz the LNA combination shows its strength at eliminating virtually all out of band noise!