The difference between moderate speed and fast as possible is somewhat subjective. My personal opinion is no more than 15 mph over the speed limit should be the top speed during Code 3 driving, and possibly less than that depending on road conditions, traffic, etc. Then again, that's based on the conditions my department faces, where we're usually looking at a 5 minute max transport time because of our proximity to the hospital. I might feel differently in, say, a rural area with extended transport distances.
Still, I don't think you should drive any faster than you can comfortably drive and control the ambulance, and it should be based on road conditions, traffic and setting. A straight, open road with little traffic is different than going through a town with cars and pedestrians, as is driving on a wet or icy road.
So, I voted for moderate speed with RLS, with the understanding that my definition of moderate might be different than yours.