Just to say Fishy, if the writers of Exar Kun, had constantly depicted him as shooting out blasts and destroying people, do you honestly believe that it would make a interesting read? Hell if it went "kun blasts muahahahahahaa, he pwnzors" it'll be getting heaps of hate mail. Same reason, why didn't Luke use his emerald lightning constantly? With your logic it means he cant, he did it once, and he never did so again, so that means he cant do it? It means that one, he didn't feel there was a need, or two the writers of Luke, did not want to depict him reusing the same feat over and over again.
Yeh, Luke and Sidious have had more training, why? because they are both much older than Kun himself, whats to say Kun wouldn't learn more or reach a higher potential if he had not died? Your logic is pointless, as well the sources on both Sidious and Luke surpass the sources on Kun by a astonishingly large amount.
Consider Also Kun, had absolutely no equal during his time, it took thousands upon thousands of Jedi to force him to entrap himself, he never had to demonstrate amazing feats, he was the strongest of the time, he had no-one to even remotely challenge him, having tooled the greatest of the Jedi in his time, completely humiliated the Senate on Coruscant.
Instead of proving what Kun can/can't do, why not try proving that Sidious and Luke can replicate the feats on which you believe can destroy Kun. NJO Luke using green lightning? Sidious using his force storm? Prove up, just because Sidious did it once does not mean he will do it again, so thereby unless you can prove Sidious and Luke can replicate their feats, you have defeated yourself with your own logic.