I have tested the maximum switching frequency of the ports in MKE15Z7. For this I used an FTM with an output signal placed on one of the doors. With the clock frequency of the module at 72Mhz I managed to reach up to 3Mhz at the port with a reasonable signal.
Is it possible to further increase the frequency of the output signal while maintaining the electrical characteristics?
I think the short answer is "No".
We had a discussion regarding ths a couple of months or so about it and there are a lot of reasons why direct DO is limited to 100s if kHz when you want to do something intelligent. If it's a repeating signal, there are tricks like using the SPI ports that will give you speeds up to 10Mbps or so.
What are you trying to do?
myke