Is it top "sustained speed" or "peak speed"?
I do interval training with GPS just to monitor my effort vs speed ratio, and so far the highest peak speed I hit during an interval sprint was 18.7 km/h, but it was only for 5 seconds or so and then it droped to ~15 km/h which was sustainable for 1-5 minutes on average.
By the way, I paddled on glass on a club OC-1 (+35lbs) with a rudder, so I assume anyone with a better or a dedicated sprint OC-1 could do better.

On my GPS I average about 7.5 to 7.9 mi/hr for 8miles. The max speed I've hit in the flat is 9.7mi/hr for a few seconds. I paddle a Kaimana (24lbs.)

You can look at the archive of the Oahu short distance season at the Kanaka Ikaika website/archives. You will find the race results for the last years for the long courses and for the short courses ( beginners ). The races are not flat water, so the surf helps at times.
Windward Kailua 4 mile race had a winning time of just below 40 min, that makes it about 10 min/mile. So that would be 6 miles/hour race speed.
So if you paddle at 8 miles/hour as a Nov A or Nov B you will probably win that race. This would be about 7:30 min/mile, and if you look at the race archives, beginners don't do that often, I guess they can with lots of surf.
It is interesting to see that the established paddlers finished 8 miles in 40 minutes in Kailua 2004; their course goes around bird shit island and they have a longer downwind section where surf also helps. Still quite a difference in speed.
On the Fusion in absolute flat water - Kailua canal - we just did about 9 min/mile without specific preparation with 85 - 90 % effort.
Should we be able to bring that down to 8 min/mile, we would look very good over 4 miles. 8:30 min/mile seems realistic as a first target, averaged with and against currents/tide in flat water.
A new thread discussing the use of GPS would be interesting; I think first hand experience and comparision with a Speedmate would be nice.