with that shaped tube it's not really representative of the potential failure you would see with a round tube ISP.

I always wondered what Rob English's failure rate was like. I see production bikes that were not made to be weight weenie bikes fail all the time in the top tube/seat stay area. And I hear custom builders talking about failures in the places where he has no reinforcements. Admittedly, a lot of those are on mountain bikes.

I have thought about doing an ISP, but they generally weigh more and are more expensive than a standard seatpost arrangement. I would never do a lugless bike without a reinforcement at the top tube/seat stay junction, but even that might not be enough for an ISP.