Pete and All,
 I have used excluders on some hives above two deeps. They still put honey in the supers.
Last summer while I was in Ireland I saw a negative affect of a single box covered with an excluder with supers on top. The beekeeper told me that the hive had swarmed 3 times so far and it was in June. We gave the queen more space to lay and it worked out Ok. I could have been the date also.
 All bees and queens are not the same so its not one size fits all. The beekeeper needs to watch to see if the brood nest is all plugged up with no room for the queen to lay.
Best
Roy

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