I do see certain flaws in your reasoning/calculations. Whether they are fatal or not is another question.
Before I go into those, I will just mention that I love waterwheels. There is a certain majesty to them that I do not find in a turgo or pelton runner. However, a Turgo runner should have an efficiency of over 90 percent if properly installed in an appropriate application. (your 60 percent figure most likely is a system figure, turgo plus alternator) So, I am not sure how you expect to design a system based on a 80 or 85 percent efficient water wheel and beat the turgo.
Your horsepower figure of 5.78hp is too high. 100L per second is 1585 gallons a minute. One of my references (for an 85 % efficient wheel ) lists 3.22 hp for a 10 foot (3 meter) wheel using 1500 gpm. According to their figures, you would need an 18 foot (5.4 meter) wheel to produce 5.79 horsepower with that flow.
I think your error was in assuming that you could get 3 buckets worth of water at full radius. I don't know how many buckets you are assuming, but let us use 12 as an example. Draw a clock face. Your water comes in from the left and spills into the bucket at 1 o'clock. The wheel will rotate clockwise. Draw a radius line from the center of the wheel/clock to the bucket in the 3 o'clock position. This is the only full bucket with a full 1.5 meter radius lever arm. Now draw a vertical line connecting the 2 and 4 o'clock buckets. This vertical line will intersect the radius at some point roughly 1 meter from the center point. That is the effective lever arm for those two buckets. If you draw another vertical line from the 1 and 5 o'clock buckets, it will have an effective lever arm of roughly .5 meter and the weights are reduced because 1 is filling and 5 is spilling. So it is essentially impossible to get 3 buckets to full radius lever arm, on a wheel of this size.
(I also am missing the correlation of how based on a 100 liters a second and a 10 foot wheel, you developed the figures of 4 rpm or a bucket size of 100 liters, but will base my calcs on those figures.)
The largest error is your counting magnets and coils to leap to an "effective 1604 rpm".
If your small motor/alternator that is turning 1000 rpm has a 5 inch diameter rotor, the circumference is 15.70795 inches. At 1000 rpm it is 15, 707.95 inches a minute or 261.7992 inches a second. This is your "coil/magnet passing speed". Assuming that you mount the magnets/coils near the rim on your 10 foot water wheel, I came up with 24.72071 inches per second "passing speed".
If you would rather count impulses, in the alternator you get 12 magnets under 12 different coils or 144 impulses per revolution. 1,000 rpm is 16.6667 revolutions a second so 144x16.6667 equals 2400 impulses a second. On the water wheel you have 12 coils and 401 magnets. In one second, you will only get about 26 magnets passing under each coil. 26 times 12 equals 312.
So whether you look at "passing speed" or counting impulses, the water wheel mounted coils/magnets are not going to have an "effective 1604 rpm" or come anywhere close to output of the small alternator turning at the high rpm.
Mountainman made the comment that larger rotor equals higher passing speed. That is true if you are driving an alternator from the shaft, at X rpm. With waterwheels they are driven on the circumference. So as a rule of thumb, two different sized but otherwise identical wheels driven by equal water flows, should have identical "passing" or rim speeds. So, with identical rim speed, the smaller wheel (smaller circumference/radius) will have a higher rpm than the larger wheel.
I would actually encourage you to build a water wheel or several of them. The experience of building/operating water wheels should give you a wonderful appreciation of several thousand years of seat of the pants engineering. They are a much more refined machine, and far more complex than their simple appearance.
The best reason for building a water wheel driving a generator is simply because you like a wheel. It is going to be less efficient than the more modern turbines. But with enough water flow, it can produce enough electricity. So, assuming that the excess flow is available, to offset the lower efficiency, do we really care ?
The "wheel" has that majesty or a certain ambiance that a turbine just doesn't have.