I would also like to add my frustration with the workout creators max of 300% FTP for intervals. I have recently started to focus again on some sprinting drills and have butted up against this limit. In my own case I’d like to make a series of sprint intervals in the range 200 to 500% of FTP.
In the end I was able to make the workout I was after in Zwift using the ZWOfactory web-based workout creator. I have a Kickr and found that I was able to get good transitions to relatively high-power short intervals (>= 500%) if I included a short ramp interval of between 10-15s beforehand.
I would hope that the 300% limit is some arbitrary value (possibly hard-coded ) in the source that could be increased rather than some fundamental limit.
I have used the workout creator to construct many of my own custom workouts over a long period of time and hope that such an improvement could be made as it is somewhat of a hassle to switch to a different app for the workout.