It is possible to see Masters in Pro tier comps though as it happened to me.
I played the first round of a comp as a Pro, then made master a little while after. Obviously I can then buy Master rated equipment but had to finish the comp in the Pro tier.
I just wanted to make it clear that when playing in the aforementioned comp I had to change my kit back to Pro kit for those particular rounds. That's why it looks like people are playing with Master kit in Pro comps. Once the round in the Pro comp is completed they change their clubs back the Master ones.
I hope that makes sense.