Nice build, even though my Exige has the factory SC, I prefer a well engineered turbo setup.
Regarding the non-linear throttle behavior, what are you using for boost control, electronic or manual, and where are you taking your boost reference? One thing you might consider: taking the pressure reference for boost control from the intake manifold can result in excess air pressure (above your boost target) before the throttle body when you are at partial throttle acceleration, and this will contribute to the non-linear throttle behavior. This isn't an issue if you're using a manual wastegate controller or no adjustable boost control, provided the feed for the pressure signal is near the turbo outlet.
Maybe you have a good point. The Hondata control the boost with a basic 3 way air valve. The pressure reference is into the intake. The set point is 14 psi into the Hondata, and the engine was running at 13.6 psi on dyno. It is easy to find good reference talking about the advantage and disadvantage of the three possibles place to pick-up the reference pressure. For the non linear, whatever where thr pressure is picked, at low throttle, the boost with be lower than what the controller is helping, maybe 6 psi. But at 6 psi with the TB at around 25% opening, it is big power with a very sensitive pedal. It is like controlling 300hp with 50% pedal travel. Again, it was not very bad, but not perfect and asking care.
All anti-lag parameter are off on the Hondata. The turbo lag is very small, and the transmission is a close ratio. The engine rpm drop is very minimal. At the beginning, std parameter for the anti-lag were programmed, working great on dyno, but not at all on street. I was all a time trigering the safetu boost limit. With anti-lag off, with the reference point into the intake, after quick shifting (3-4, 5-6, but not slower 2-3 or 4-5), the instant increase of boost may trigger the safety limit. I increased the safety limit to 18 psi if I remainber right.
But for sure, the turbo set-up gives big amount of torque in mid-range, like a V8 muscle car.