If you look at the first picture in the thread, you'll see where BW says it spools almost identically to a 6758 and makes 50hp more. That was back in 2012 and they've since done more R&D and upped the rating by 50hp. So that's where I'm getting the 6758 response with 100hp more thing from; I just hope BW delivers on that.
Also, how much of a difference are we talking here? 5-10hp if the gate cracks a bit? If I were to get one, I'd go twin scroll and at $200+ x2 for external wastegates, the internal option looks really good to me so long as there is not a significant impact.
I didn't find any comparisons in power between IWG and EWG EFRs but I did find an example showing that the IWG of an EFR flows better than an EWG on a Garrett.
GT3582R vs EFR-8374 - Undivided T3's - Back-to-Back Dyno K20 - Honda-Tech
The Garrett crept while the EFR did not. I realize creep is not the issue you were discussing here but this is all I found that compared internal and external.