At the onset, allow us to thank you for guiding us to this new FE enhancing techniques. Our knowledge on this was Zero before your query. Post receiving the same from you, we did read a lot about it, what seems to be obvious that as a tech, if used correctly, it does improve the fuel efficiency of the car. So in-effect you might already know whatever we share with you.
We did not come across any major car related issues (but our reading and knowledge on this is limited) but we did come across a lot of safety concerns. Most articles on Hypermilling did start with a caution of using it with lots of care.
Slowing down on turn by slotting into neutral could be dangerous if you are entering a turn requiring a slower speed at higher speeds, but basis, what we read, you seem to be ensuring that you do not do this mistake. Yes we have read about using the car's roll into the turn and then taking it up by using a higher gear, however, you need to take the next higher gear rather than skipping gears.
All cars in neutral will go down to its idle RPM, nothing unusual about it. The trick in such case lies in revving the car up gradually and not by flooring the pedal. Gradual pick-up would cause no damage, however, flooring could cause harm to the engine.
Are you driving a diesel car, because in case of petrol cars one does not have to rev-up to 1800-2000 Rpm to get the power flowing into the car. Diesel car do have turbo's clicking in around the RPM band mentioned, till then you do experience a diesel lag.
If you google hypermilling, you will find many more and probably safer ways of enhancing FE of your car.
Our request and advice, use it with lots of care and caution. Never do the mistake that I did of shutting the engine on a slope and banging the car in front. Learnt a Rs.20000 lesson.
Please do take our advice with lot of caution and read more on it. As stated earlier we have also learnt about this technique thanks to you.