So a old guy such as myself that can hold a 4 to 4.5 mph pace in the petrel will not be much if any faster in the mystery assuming I am exerting the same amount of force.

If all you are want to do is cruise a little faster, you probably won't benefit much from a "faster" boat, but if you are racing and willing to push hard to go fast, the Mystery would be faster.

As an example, the Blackburn Challenge is 19.6 miles. On an average day out for a paddle I would give myself about 6 hours to do a 20 mile trip in a Petrel or Mystery. This would allow time for lunch or snacks along the way. If I were to do it non-stop at a casual pace I could probably do it 4:30 to 5 hours in a Petrel. If I were in the Mystery, at a casual pace I might take a few minutes off the time, but I would probably stop every once in a while to rest. But I would finish the trip feeling fine.

Given a race situation I can do the distance in less than 3:15 in the Mystery and I will feel wiped out and take days to recover. Racing the Petrel it might be 3:45 or more and i would be tired but recover fairly quickly.

Going fast takes more horse power and sustaining the energy level takes more out of you. The best athletes in the Blackburn do it in 2:30 in narrow surf skis. While these fast boats make the speed possible, the level of fitness required to maintain that speed is amazing. It takes a tolerance for pain that most people don't have.

So, if all you want to do is casual paddling getting a "fast" boat will only have so much effect. If you are currently paddling a 10 long design, going up to a 14' design will be noticeable. But, adding another 3 feet will make less difference. A 17' boat is only a little faster than a 14'. Paddling my Petrel Play, I can keep up with 18' Explorers just fine (until they sprint).

Adding another 3 feet probably won't make any difference. 20' boats need a lot of power to get into the faster zone. And to the extent that the longer boat is narrower and less stable you may even be slower in the long boat because capsizing is notoriously slow.