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I've got a tester one that is lower than that if you want? had it on my caad10 so I know it works, one of his slamthatstem ones.
Regarding your question, it's a very personal thing, the bikefitting thread will get you lots more opinions but in summary:
Speed - you'll be lower so in theory more aerodynamic so yes, but you may not have the flexibility for it so not engaging all your gluteus and quadricep muscles, then no. then you might become so uncomfortable that your breathing is affected, again slowing you down
improve position - bikefitting thread, video of you on rollers, see opinions, or get a pro like @scherrit to see you
comfort - again personal, under the higher-you-are-the-more-comfortable-you-are ethos that bike manufacturers seem to have bought into then no, but it's personal, there's a theme here isn't there...
This'll play nice with a CAAD10, right? Just got one in the post, and considering lowering the stack height 2cm by replacing the 25mm conical spacer with this 5mm:
http://www.cannondalespares.com/blog/2013/12/slam-that-supersix-stem/
I'm 6ft4 / 193cm on a 60cm frame with a 59cm top tube and 120mm -7deg stem. Any thoughts on lowering the stack height and if it'll slightly increase speed / improve position / decrease comfort on long rides, etc?