matrox triplehead2go + soft TH works (I saw movie on youtube yestareday)
guy has 5 x screens
on matrox 3 as 1 in extended mode and 2 on sides
so system saw 3 physical monitors only - |side| |main| |side|
so matrox + sofTH works, so in theory eyefinity + softth should works too
but I think I will be pionier in this , need only to wait for active dp @ dvi adapter
I was the original person to initiall do Th2G+SoftTH. I can tell you right now Portrait mode won't work on Th2G. Your theory will work up to 8000 or 8192 pixels wide total but not wider. On the other hand, EyeFinity on 3 screens portrait and SoftTH for the outside 2 "might" work. You'll have to be the first to do it like I did with Th2G a few years ago. There always has to be someone to take the first step with exotic display configurations.
Personally, I'd just wait for the 5870 EyeFinity6 six port card to come to market. Keep it simple, don't limit yourself by using something obscure. EyeFinity+SoftTH is way too obscure to get proper support with anything.