Are you guys saying that a T3 can't produce any boost pressure above 5500 rpm on a 2.5L engine? The problem may be the turbo is too far from the exhaust, and efficency is lost because of heat loss.
Or, the small turbo simply maxes out. I would be very concern about having a tiny turbo. It actually IS a lot more stress to the drivetrain and engine than bigger one (of course depending if you go higher with boost, but even with the same amount of psi, but please do notice that with bigger turbo the same psi isn't the same amount of air flowing into the engine. It's more.) and it WILL have more restriction on exhaust, which leads to backpressure issue, which leads to a possible overheating issue.
From what I've experienced during my few years on boosted KL03, I wouldn't bolt on anything smaller than a decent sized T3/4 hybrid for thoughts about even +50% power increase without having to worry about the engine stress. That goes even for low boost. Always, and I do mean always, design your system so it will support somewhat bigger amount of power than it is. That way, you might have your boosted Mazda next week too.
That's my thinking and my point of view, other may think different. What I can say, I've had my car turbo'd for 2 years soon, and I haven't blown the engine yet. *knocking the wood*
I'm all excited to see what happens on the dyno. I have butterflies on my tummy, telling me that something might break up... We'll see.