I have only received my KPA1500 recently, But I decided to put it through the paces with the K3S being the exciter.
I found that into a Cantenna Dummy Load that has not been tested for 50 Ohm impedance that below is what I get from the KPA1500 Display and a LP-100A meter, but never really used the LED's as the indication of power.
20m - 10m approx 30w output on K3S yield approx 1425 -1475w Output
40m - approx 27w output on K3S yield approx 1450w Output
80m - approx 25w output on K3S yield approx 1450w Output
30m and 160m antennae are only rated for 1kw, so didn't do the test on those bands.
Mileage will vary from station to station, and as Bob mentions the Dummy Loads actual impedance is a factor, but so far I am happy with the KPA1500 performance, and see no need to go out and get a lab spec Dummy Load for something I rarely use.
73 De Mike