Speed relative to other moving traffic is only one risk factor and, I would have thought, a relatively minor one on the average urban commute. There is a much higher risk of accidents from unpredictable slow moving objects than from relatively predictable traffic movement and as traffic gets faster it does behave more predictably, making it easier to avoid. Car doors, pedestrians, dogs, potholes, diesel, debris, other cyclists etc. are constant threats. If I'm right it would be safer to travel at nearer the speed of those things, or at least speeds that mean you can avoid them.
Just to avoid doubt here i'm talking about the likelihood of accident rather than severity. Contact with a fast moving vehicle may be less likely than hitting a dozy jaywalking pedestrian, but lots more painful/fatal if it does happen. I can't imagine that the damage caused by being hit by a car at 50mph will be much different for a cyclist whether they are doing 10 mph or 20 mph though.