I'm glad you posted this I was just looking at this stem from FWB, I plan to use the Zipp SL handlebars also. What was Nm used?
The on-line specs for torquing (which I can't find today on the Extralite web site, but downloaded to my computer about a year ago and will send to you by PM if you wish) are that "5Nm maximum torque be applied to the stem-to-steering tube bolts" (not 2.5Nm as the print version states) and "the stem-to-handlebar torque" is listed at "3Nm" as compared to 2.5Nm in the printed specs. I used 4.5Nm on the stem-to-steering bolts and 3Nm stem-handlebar and I've had no problems.
According to Sergio (Extralite) the issue with higher torque on the bolts is "stress corrosion". Here is what he wrote (excerpted from an e-mail to me from Extralite):
Please note the following:
1) Fork steer tube should be round at the diameter of 28.55mm (+/- 0.05mm).
2) 5Nm is the maximum allowed torque, it's safer to release it during winter or when you are not using the bike for a long time.
Notes about "stress-corrosion"
This phenomena happens to high-strength alloy when the part is constantly subjected to an excessive tension and this lasts for a long time (the excessive high tension is normally produced by an excessive and/or uneven bolt strain).
Additionally storing the bike with the tensioned part in a humid place will significantly speed up the stress-corrosion process.
After the "stress-corrosion" starts to show its signs the part is not repairable and its not usable anymore.
The usage of the bike and the part is not a cause of the stress-corrosion, the reasons are excessive tension and storage in a permanently humid environment.
And here are some comments about the issue of "stress corrosion" of these bolts (as mentioned by Sergio of Extralite in the above comments) from a mechanical engineer/cyclist:
"I think he is referring to micro cracking at the molecular level which can allow the "skin" of anodization to crack enough to expose the underlying raw material to the elements. When this happens it can corrode (oxidize) which as the material is thin, can be a problem causer over time. I wouldn't worry about it, simply keep your bike clean and make sure you inspect your bike regularly (which you should do anyway).
Remember that any clamping part like this is effectively working as a spring, it's under tension to hold parts in place. What he suggests is to let the spring back to it's untensioned state during periods of disuse, to avoid it getting a memory (which means you could tighten it more and more over time potentially until it cracked over the years). The margin of safety is outside of this happening, he's just being cautious."