Boost is nothing to do with flow.
Far as I understood SRs could happily do 450bhp on stock head ports anyhow, so it can deffo flow better than many 2ltrs.
As said before, regardless of engine/turbo/whatever, peak boost (it don't have to hold it) to get about 400lbft from a 2ltr is generally 30psi or more.
That recipe is the same be it a YB or an Evo engine, both of which commonly do it.
I mean, for example, on a Cossie it tends to be the same thing that's been done to get 400bhp/400lbft for the last 15 years- a oldish skool T34 turbo, mildly ported head and cam(s) (as stock heads dont flow great, less than an SR one for example), 7.5-8:1 compression, enough fuel and cooling to do the job, and 30-35psi boost (PEAK, for the big torque- Tends to drop to ~25psi by peak power), job done.
The key is making something strong/reliable enough to do such things. Shouldn't be hard on an SR, they're not weak. I've seen a stock SR run 2bar before and got about 400lbft if I remember right, but it didn't last long, unsurprisingly on 8.5:1 cast pistons on pump fuel.
Getting it isn't hard, just need to spec the engine and turbo to suit what you aiming for.
The problem with 99% of engines is people pick parts of loads of different setups they see working well and create an engine from it.
That's ridiculous, you need to decide what you want your engine to drive like and what fuel you're using, then work out what kind of things will allow the engine to drive like you want on the fuel you will be using, and then spec the turbo and engine from there.
A mish mash of parts people see performing well just makes for a pretty ordinary engine usually.
