1990 Nissan 200SX SR review from France

Recently my car seems losing power when revving between 2500 to 3000 rpm(slightly before the turbo engagement range). this problem magnifies especially when changing from 2nd to 3rd gear with low rev. It's weird! I feel normal power output below rev 2500 and turbo output above rev 3000. (what happens in-between?). So I took my beauty to a nearby Nissan dealer for a cure. Well, according to Nissan's guys it could be an AFM problem, airflow to the turbocharger might be cutoff abnormally when the AFM is in bad condition. But they can't guaranty after changing the AFM

my car will regain its normal power. Moreover the AFM is extremely expensive (it costs around 700 euros in France).

Anyone out there experienced this problem? I need advise before changing the AFM or other parts blindly. Thanks for your help.

Nothing more to praise, the whole world knows what the Nissan 200SX worth, beauty, comfy and speedy yet under-rated and unestimated compared to other GTIs What's a shame! I'm lucky enough to own one, and you too!.

My car is currently stock except 17" wheels, 235/40 tires, K&N admission kits, Remus exhaust, momo steering, knob etc.. Is it wise to up-rate my engine with such mileages?. What do you think?.