Pete,
The hulls from these show ski boats look totally different than the pictures of the single rig 17ft 6in and 20ft boats you showed before, are they different or just stripped down for show ski use? There is a local show ski team in my area that runs a twin rig and a single but I believe those hulls are at least 20 years old. I am sure I could get a ride behind one but I don't want to give you obsolete feedback as I assume things have changed quite a bit (they do have newer motors).
I have driven the show ski boats as well (again 20+ year old hulls with newer motors), but this was 12+ years ago, but I found the twin rigs were harder to get a constant speed out of than a single rig. The boat either wanted to go 15mph for a swivel skiers or 45mph for a barefooter. In between there to get it dialed in to exactly where you wanted within .5mph could be a little challenging. Could be that there was so much power available it is hard to really fine tune a speed. Could also be an older hull, and older controls and things are easier now. The wake on that old twin rig was less than desirable for slalom skiing, but those boats were weighted to keep from flipping, the second engine adds a lot of weight, and two props makes for a very different table behind the boat.
As for the swim platform size I would take a guess that it at minimum needs to be half the size of my nautique platform. I'll go get a measurement this afternoon for you.
If you find yourself down in my area (la crosse) I'd love an opportunity to ski with you in the course. I think you are making some really cool boats and there has been huge advances in outboard tech that most of us are totally unaware of.
I could get a ride and drive the local ski teams single rig (or twin) in the next week but I think your boats have changed enough that I would not be giving you anything valuable.
I will say with the abuse show ski teams give your boats they do seems to last forever with the hulls seeing multiple motors over their lives.