Single exhaust & single turbo

6mt370

So I'm less than a month away from getting tuned on my BP kit and I have one question for you guys. I have a tomei single and iv seen the thread where Z1 makes 600whp on an AAM TT kit with that same exhaust and that's on 93 oct . My tuner says I'll be hard pressed to get 500whp reliably out of that exhaust. I called Z1 and they said that the tomei exhaust is actually an extremely good flowing system and actually measures a little bit bigger than just 3 inches, the rep said its about 3.1-3.2 ID. I'm also going to be strictly using e85 fuel. So I'm not too sure how there could be two different sides to what power that exhaust can make. I don't want to throw my tuners name out there but imo he has an extremely good reputation and I have honestly never heard a single bad tune or anything coming from his shop. He's a stand up dude and has been with me and my build step by step and he's taking it like his own. I do not have the $1500 to switch systems and my only other option at this point is to get a cutout installed right after the turbo if I wanted to make any power. What do you guys think? I'm not chasing numbers but I wouldn't be too happy if I only made mid 400whp when I could be making a hell of a lot more. Opinions are appreciated and if anyone has actual experience with this exhaust on their turbo cars I would love to hear it.