You mean this one?
Looks to me like due to aerodynamics, this thing will slow you down at speeds over, I dunno, 15 mph perhaps. In the long run it may be worth it though, if you can access whatever you need without stopping.
I don't think weight will have much effect on speed or handling unless you really load it up.
Something like the Topeak Dynapack
might be a bit more aerodynamic, but I'm a little bit skeptical unless someone has actually tested it out. And again, to get anything out of it you'd have to stop, which may kill more time than you get back from the more aero design.