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Hi tommelcher,
With that kind of traffic you are going to face scalability issues with PHP before you even get to a CMS, purely because PHP is an interpreted script rather than being compiled. Added to that you are going to need a massive amount of bandwidth to support 100,000 concurrent users, you'll be looking at something like a dedicated OC-3, maybe even an OC-12. No doubt you've taken that all into consideration though.
Anyway, throw enough processing power at web application and you can acheive almost anything. I'd assume with the kind of traffic you'll be looking to support that clustering your service, which is something I can honestly say is an environment I've never heard of XOOPS operating in. However, effective clustering is a function of your clustering solution rather than your choosen CMS.
The largest site I know of using XOOPS in any detail outside of Xoops.org is iis-resources.com, but even then we only get around 250,000 page views a month there which is no where in the league you speak of. I can remember reading about a drag car website some time ago that had 500,000 visitors in a weekend, but can't remember any details.
Funny you should mention the article regarding the analysis of 4 XOOPS article managements systems, as I wrote that paper. I'm actually in the middle of writting another paper on performance tuning your XOOPS environment which deals with performance optmisations in a single low end system environment from the operating system level right through to web server and web application level in a Microsoft environment. I've gotten some pretty impressive results so far, with performance improvements in the vacinity of 1000% over the default. Even so, this would be of limited value to you as it only covers general performance tuning methods, where as you will need to do some significant performance optmisations at the web app code level I'd say. Anyway, I'll be very interested to follow your progress here.