I agree about your comment regarding diodes if we were using conventional silicon diodes with a 0.7V turn-on voltage.
We use transient voltage suppressor array diodes with a breakdown voltage of 6 V so it has little effect on the measured OIP3 either on the input or output.
Obviously I don't want to give away too many of our trade secrets on a public forum but if you are interested in knowing more you can always contact me by email.
