Steve - I'm not familiar with the Screen Excellence material, they say its proprietary, but they look to be woven fabrics similar to Phifer Sheerweave which is used by several boutique manufacturers. I have a SeymourAV screen and love it. Mine is retractable, but he does now offer a very nice fixed frame screen, as well. He also uses a proprietary weave in his fabric now although he started off using the standard 4500 fabric from Phifer like many others continue to use. The nice thing about the fabric is that it requires no EQ (unlike many perforated materials including Stewart's) and it has a negligible effect on frequency response or attenuation. You can place the speakers only inches away without any adverse affects. I don't recall what the recommended minimum distance is but mine drops down in front of my Elite RPTV and my center speaker is only about 3-4" behind the screen. I think the main concern is really air movement from any large drivers or ports that can cause the screen to move.
All screens have some compromises and perhaps the two most common to acoustically transparent fabrics are the texture and the low gain. The texture can occasionally be visible if you look for it but I find when I'm really involved in the program, I hardly ever notice it. I will say that using my RS1x on the CenterStage XD material I have the texture seems slightly more noticeable with SD material, but not hugely so. The XD material has a gain of ~1.2 but the RS1x has plenty of output at my throw (13') and size (104" diagonal.) Also a major concern is moire if you plan to go with an LCD. This will vary based on screen size and throw. If you go DLP or LcoS (DILA/SXRD) then its really not a concern due to the extremely close pixel spacing.
You can checkout the SeymourAV screens (hand made with care by Chris and his crew in Ames, IA) at his website
. I also encourage you to head on over to the SeymourAV screens thread here in the forum. In addition to his great products, Chris's customer service is second to none.