I have had GT version done in my engine and I was thinking about air filter + pipes or some special pipeline ( I don't know its name, probably jet pump or ejector) but after some examination in the garage decided to skip it, because the cost compared to profits would be too high. And of course you need a skilled mechanic to fix all the pipes properly. Without those pipes all sports air filters are almost aways useles. In Poland many young people start their mechanical tuning with air filters, magnetisers etc and the results are easy to guess. That is why I suggested leaving the the idea.
Calculated profits for my car were:
basic power: 41hp
after GT:50hp
after air filter + pipes: 51,5hp
As you see the results are not very impressive.
But I beleive that you are not angry with me.
Zdrówko
P.S. What are NA cars?
Does skyline means cars after tuning?