Thanks for the responses so far.
I am interested in how much power you think you would gain by having a stiffer stem. Or put another way, how much power you think you lose by having a flexy stem.
Hornedfrog hit it on the head; for me it's about handling, not power. I currently have a Deda Zero 100 with Thompson KFC bars, and since I like the shape of the bars I do want to stick with them but would consider changing to another -8 or -10 stem if there would be a noticeable improvement in stiffness from that component.
According to Fairwheel's review the Thompson X4 is the stiffest alloy stem, beating out the Pro Vibe 7s in the deflection metric, with the Pro Vibe 7S having the edge in stiffness to weight ratio. While this is WW, I am open to adding a touch of weight (gasp!) if the stiffness gain would be noticeable.
Would be great if someone could give a real-world based comparison of the Deda (currently on my bike) to either the Easton or the Thompson.
Or for aesthetics of matched components, I could just go with the Thompson stem based on the Fairwheel tests... although I'm not going to replace my Syntace HiFlex P6 seatpost with a Thompson....