I've sailed in a lot of different conditions and with different crew and I don't think I've ever measured anything above 17. I can hit that pretty consistently. Perhaps I could do better solo, but I rarely sail solo because I don't weigh enough to right the boat on my own. I have sailed with crew trapped out (far out because I have wings) and with me on the wing seat and still enough wind to need to let some spill to keep from going over.
Something I found when I sailed my Hobie 17s, is that power doesn't always equal speed. I used to trap off the wings and sheet in tightly so that I could fly a hull while trapped out; great fun! But I sailed fastest when the boat was pretty flat. Battens relaxed, downhaul and outhaul stiffened up, leach line loose, mast rotator pulled tight, and the sail angle kind of loose so the tell tales on both sides flattened against the sail. The windward hull would be light in the water, but little chance of flying it. That's when the 17 would just rip through the water.
The Getaway doesn't have the sail tuning the 17 does, but for speed on windy days I pull the downhaul tight, loosen the battens (to flatten the sail) and put the main block in the forward hole. I sheet in tight and control sail angle with the traveler. I keep the jib slot open so the mainsail gets the cleanest wind over it; I need to add jib travelers one of these days - I think that's probably the weak point of the Getaway design. I think more mast rake makes it faster too, on windy days, but I don't bother with that. I did on the 17, but going fast was more important to me 20 years ago!