I wouldn't say nuts, but you will notice a difference in the handling going from 120 to 190 lbs on the transom in a 4.2m boat. It will not feel as nimble and responsive. I have had a Yamaha 40 hp 2 stroke (weight about 170 lbs) on my Mark II grand raid. It was faster than with the 25 hp 2 stroke I normally had on it, but not nearly as much as I had expected it to be.
I haven't used either a durajet, or a jet leg, but have read that (compared to a propellor) the durajet should provide respectable proplulsion (with some loss at the top end) whereas with the the jet outboard, there will be significantly less propulsion.
Are you looking at these two drive types because you are planning to use the boat in some sort of rescue application? That is where I can see those drives having the most practical application. I believe the main reason the military uses the durajet 55s (currently 55hp ETECS that weigh 240 lbs) with their FC-470s is for the safety of their personel who are regularly extracted from the water, often at some speed.
If your application is more for running up rapids & going through shallow water, a regular prop with a prop & skeg guard is probably more practical. It's easier to carry an extra prop or two than try to repair a specialize propulsion system in the field.
The durajet looks interesting, I've looked into them, but have shied away, because I was leery about downtime and costs if it required parts and repairs.