It's old adage - if something seems too good to be true, it usually is. The GTX3071R must be, what, £1200ish on it's own these days? I paid £1000 for the much older tech GT3582R way back in 2008ish.
Full boost around 4000 is a bit too late for my tastes, but the top end must be insane! There is a benefit in that the car will already have a lot of momentum by the time 4000rpm comes up, so the boost hit isn't so hard on the gearbox, or tyres. And the flow of a GTX is more like a big shove in the back, compared to a spikey little punch.
What can be done with mapping to overcome a physical limitation? Apart from N75, throttle and timing, there isn't a lot really. I've heard of this 'TIV' mapping to speed up the onset of boost, but I don't get it. How do you boost cylinder filling without variable lift and duration? The intake VVT on the EA113 is purely for NOx emission control I thought?