I went out last night for a couple of full throttle pulls just for the hell of it.
My boost gauge in second wouldn't go higher than 1/4 of the way up and slowly dropped after 5800 rpm. In third the boost guage would go all about 3/4 of the way up but again slowly dropped to about halfway from 5500 to 7k . The car didn't seem to lose any power and it did sound like the turbo was spooling although it's tough to hear.

Am I losing boost or is it just my faulty stock boost gauge?

Mods are S5 street ported 13bt. Hollowed out exhuast.

 

Do you have a boost controller?

Have you ported the wastegate on the turbo?

 

It's pretty normal for boost to fall off towards redline with a stock turbo.

 

no boost controller and haven't ported the waste gate at all.

 

confirm with an aftermarket gauge first but like it was said old stock turbos get tired near redline

 

The compressor wheel on a stock turbo is just not large enough to pressurize the volume of air the engine needs at high rpms. Most will be able to pull off about 6-7 psi by redline in 3rd or 4th gear, which is hardly noticeable if running stock boost (7.5psi max on S5s), but it's very clear when the peak boost is turned up and you have a big spike in the mid-range that declines all the way to 7k rpm.

Arghx just posted stock turbo maps (or very close estimates) that show this. The typical range for a rotary is 35-40 lb/min of airflow, and you can see the compressor efficiency range is well to the left. https://www.rx7club.com/2nd-generation-specific-1986-1992-17/stock-turbo-wheel-size-818137/