I would agree the trend in the industry nowadays is going for fuel efficiency not displacement, however, it doesn't mean they have to bring down the horsepower.
Bigger engine also cost some issues such as weight and actual cost to produce and technical difficulties.
It won't be surprised to see the next gen of engine goes smaller in displacement but using Force Induction to add horsepower, in that way, it gives more flexibilities to the manufacture to tune/adjust the power output.
It is very effective on cost control, engine development and also, for the owner to mod the engine.
In the other thread, there is the talk about the technology sharing between Mercedes and Nissan/Renault, Nissan might also look at the supercharger technology being used by Mercedes, as an option.
As a personal opinion, I don't see the next gen of Z will have a lowered horsepower, but a smaller displacement engine is possible, since the current VQ is already 3.7l, and most likely Nissan will have a brand new engine for their fleet, increase on displacement is unlikely since it would sounds like a bigger VQ, even if that's the case, it is rare to develop a 6cyl/3.9-4.0, and to develop an 8cyl is against the trend (fuel economy, cost, etc etc...)
It would be nice to see the next Z mounted with a 6cyl/3.0 turbo/supercharger that produce 375-385 horses. The engine is more compact, more power, more fuel efficiency, the cost of maintenance is low, and it is
easy to mod when compare to a larger displacement N/A engine.
In fact, Nissan already has a monster 6cyl/3.8l-turbo charged engine on hand, the GT-R engine, they could derive this engine to a smaller displacement, different ECU mapping, and use it on the Nissan/Infiniti line-up.
Sorry for the long post but I am looking forward to see the next gen of Nissan engine.
