Actually the rumored output of the 4L V8TT is very similar to G16: 500kW/680PS. A 5.7L won't get there though, too long stroke so it won't rev well at all. And turbos need to be so big they won't be responsive. G16 is designed to rev. In general large displacement per cylinder engines don't make big specific power.
One thing that surprises me is ball bearing turbos only cost a couple hundred USD more to make than journal bearing turbos and is a very easy way to improve response by 30%, yet many manufacturers would just save that tiny amount of money. Toyota did not cheap out with G16 probably at Akio's request, this is just complete opposite of the company's typical behavior.
It's quite common in today's market that manufacturers cheap out on components even for performance and luxury cars. Examples like progressive springs and monotube gas dampers, these are expensive aftermarket parts but at a mass production scale would only cost a few dozen dollars per set, and provide very big handling and ride quality gains. Yet manufacturers would package them into 'performance handling packages' that cost thousands. Same for tyres, saving a few bucks on inferior rubber and offer overpriced 'performance tyre' upgrades.
