To even things up, roles can be reversed at half time. The first teams of investigators can focus on the activity experience, the second team of investigators can focus on learning and the future. In the time allowed ask everyone to visit a certain number of 'question papers' and to record key points from their discussions. One or two people can be asked to specialise in each topic. These people remain at the flipchart for their topic and record key points.
Feedback can be to a plenary session or by everyone touring a display of the question papers.
Feedback from each subgroup in a plenary session. This can be a big yawn session unless feedback is tightly controlled or imaginatively presented. But flashy presentations alone do not solve the numbers problem - when there is a lot of experience and learning to share in a short space of time. Because I first used this method in a group of 16 people. Scale it up as you like, but keep the sharing groups small to encourage sharing of a worthwhile quality - unless all you want is a sharing of headlines.