1. Maybe they are lying.
2. Tire diameter will also effect top speed.
3. They are turning more than 8200 RPMs.
4. Your tiny tach isn't accurate.
It's cool that you have the tiny tach and GPS. Most people exagerate their top end ability or just give an estimate.
We GPS'd my metra setup at 38mph with 14:45 gearing. What intake are you running? Biturbo is a high rpm pipe so I wouldn't think that's the problem. Bigger intake and carb might give you the rpm's you're looking for. Or just go to 17:45 or 18:45 with a better pipe to make up for the lost low end.
That's all I have.