Take the same boat and put a asymetric spinnaker in a seaway (where the apparent wind picks up and the boat has to be muscled differently in troughs and as you accelerate quickly down a wave face) and the forces change a lot.

If your sailing is in inland waters where you don't have a lot of "challenges" and most of the autopilot use is when motoring without sails up, then I think the Raymarine (old Autohelm) sizing usually okay -- otherwise, I think you really need to examine what seastates and wind challenges you're planning on sailing trhough, how your sail balance really affects the helm forces, and whether you're really using the autopilot to steer the boat while you're not "on top" of the helm and ready to take-over at a moments notice.

The downsides to oversizing are generally higher initial costs and ongoing power consumption.