Yeah I noticed a massive diference in a number of ways.
1) without the nose cone I would never be able to run the motor as high as i do and still pick up water (our rules dont allow us to change the factory pick ups) , thats part of the reason the my cone drops lower than centre of prop shaft by 1/2inch to pull more water up, you do need to be carfull doing that though as one of my mates went to far and caused his boat to poupus, he re-shaped and works sweet.
2) I did take the boat out once without the nose cone and experienced bad blow out (if thats where you cant steer and feel where the motor is tracking), I found that the nose cone extends the torpedo of the gear case and helps it to track better, also a little kick on the right trailing edge of the skeg for right turn prop motors helps overcome tourque steer. I will attach a pick, look at the back edge of the skeg-not the best pick.
3) One test day I raised the motor just a 1/4 of an inch and gained 3.75mph.
I here what Geff is saying about under 60mph they dont do much and do agree with him for some boats but with all respect to him I have to disagree as it really depends on the boat, total weight of the rig, how high you want to run your motor and what you are trying to achieve - I am even chasing 1/4mph as that can win a race. I have to agree with Geff again, They do look cool! and I cant resist adding a nose cone to most outboards i have owned - even fishing motors.
Some picks of other cones I have made- The second pick is the one to look at for the kick on the skeg.