I run a warbird event every year in Kansas City, and have given this issue a great deal of thought. Our next event will be in July and excludes park flyers and flat foamies. The latter needs little explanation but the "Park flyers" issue indicates size as much as anything. We have 65" minimum wingspan requirement this year and will be going to 80" next year. The goal is to attract the larger, more complex and more impressive aircraft and their owners to the event. I have seen the development of the foam warbirds and they are getting more impressive every year.
Horizon markets their 80, plus inch T-28. I would not exclude that plane from flying as it is large and impressive.
I suspect one of the unspoken issues is the level of commitment. Someone who has a couple of thousand hours in scratch-building a warbird might be concerned about the relative flying skills of someone flying a plug-and-fly model. The risk of collision and loss from the actions of an inexperienced pilot is a real concern.
As I said earlier, I will allow the entry of the planes based upon wingspan but will discuss the issue with everyone at the pilots meeting. Perhaps we will have a timeslot for the injection molded planes. I'll keep an open mind on the issue and see how it goes.