This is an interesting discussion. There is a web site (http://www.psychosnail.com/boatspeedcalculator.aspx
) that has some nice calculators for computing hull speeds. According to the formula the hull speed of the outback is 4.8 knots. However because a kayak is so light it doesn't neccessarily follow the hard stop rules of the 1.34 rule of thumb. By using the Gerr's DL ratio the max hull speed is 7.9 knots (with 350 lbs), which I think is about the point his formula begins to fail. However if you go to Gerr's power required formula's you can see that as you get over the original hull speed (4.8 knots) the power required to propel the boat faster goes up exponentially (which I think is very true). According to his formula it takes .86 hp to go 5 knots, but takes 14 hp to propel the same boat to 7 mph, beyond that his formulas start falling apart.
I have a 2 hp gas motor that I use for emergency on my boats, and with a 4 1/2 inch pitch prop it propels either The TI, Oasis, or Revolution boats up to 4mph (3.5 knots). The boat type and size didn't seem to make much difference, I think this relates to what you described with the electric trolling motors. I think this is a factor of the prop pitch more than horsepower, hull speed, or weight (basically the max hp of the motor is not exceeded) regardless of boat length since we are below the max hull speed on all 3 boats. With a 7 inch pitch prop, the revo max speed was a little over 6 knots (probably hull speed max kicking in). The Oasis went a little over 5 knots (max hull speed is 5.2 knots) with the 7 in pitch prop, and you can feel the motor just starting to struggle as compared to the revo. With my TI running the same motor and 7 inch pitch prop, I can only get 4.4 knots max with the 7 inch prop (max hull speed is 5.8 knots). I am thinking with the TI, I have maxxed out the horsepower of the motor at 4.4 knots. The 2 hp motor just doesn't reach max rpm when on the TI with the 7 inch pitch prop as it did with the 4.5 pitch prop (HP limitation of the motor). These are all real world numbers on my own boats all with the same motor measured with GPS in normal conditions (light winds minimal waves).
Whether the boat is propelled by electric or gas, I think most of the same rules apply, basically I think you are correct that as long as you are not exceeding the max hull speed of the boat or the max horsepower of the motor, its the propellers pitch and efficiency that determines your speed on boats with displacement hulls (kayaks, and most sailboats).
Actually I think Roadrunner or someone did a test a while ago mounting two trolling motors on I think an outback. With one motor the boat did around 4 mph, with two motors it did around 4.2 mph but burned through the battery twice as fast, kind of proves our theory about the prop pitch.
I don't think my TI is capable of getting onto a plane. I have had my TI up to 17 mph on several occations ( I have 265 sq ft of sail on my TI). According to the Gerr horsepower calculator (which I don't believe) it says my sails are delivering around 100 hp of driving force. I design and make all my own sails, and figure that my sails are delivering at best 30 hp in 20 knot winds. At those speeds my boat is definately not planing. I used to design and build racing hydroplanes, I built a 3 point hydroplane that was 8 ft long and could get up to around 70 mph, thats planing. I'm pretty sure if I put a 25 horse motor on my TI I doubt it would ever exceed 15 mph (because it's a displacement hull), though it would be fun trying ( LOL). There is of course a difference between sail power and motor power, because with motors you can control prop tilt and add foils to dial in the hull to get it up on plane (even a displacement hull). So I'm not saying I couldn't get my TI up to 40 mph , but why... I would just get a jet ski instead if I just wanted to go fast.
Now as to why put a motor on a kayak in the first place. In my case I simply will not go out into the ocean without my emergency motor period. There have been several occations where my motor has saved my butt. One time I was about 7 miles out and hit a shoal and broke the rudder. I couldn't have possibly sailed upwind into a 15 knot wind to get back to Key West. Another time a storm came up on us and we were fighting 35 mph winds and 4 ft waves ( my TI can't sail upwind in winds over 30 mph). Another time I was 20 miles from my launch site and the wind died. I have around 1200 miles on my TI to date, and have used a total of 3 gallons of gas with the motor to date (about $10 bucks).
All of our Hobies have always been configured with with weighted keels, jibs, spinnakers and mainsail, actually the Revolution with the Hobie sail (22 sq ft) and a 35 sq ft convertable jib/spinnaker was probably the most fun but I never felt comfortable taking it out into open ocean (even with the motor)
Hope this helps