I'd really like to see a definitive answer...... I don't wish to spend hard cash on the steam build of Crysis if its no good for MWLL.
Crysis via Steam costs almost $60 if I'm not completely mistaken. If you've been tracking just this "General Discussion" subforum in the past few weeks, you should have seen a number of discount offers on Crysis eating Steam's price alive without chewing.
Even Amazon.com has it at $36.99 including free shipping.
In other words: go get the retail version. There is no need to take any risks with Steam.
I will not comment on whether or not the mod has been tested with the Steam version, simply because I don't know for sure, but I tend to believe that none of the people currently playing the mod is running the Steam version. I can assure you though that there have not been any intentional and dedicated tests with the Steam-Version.
Again: if you want to avoid trouble, get the retail version.