The problem is you are running a turbo that was designed for a much bigger engine, and it looks like it comes from a
CAT Diesel, some of the new turbos come with a waste gate, I can't tell from the pics you have here. Even if it did have an internal waste gate it would be set for 17-25 PSI, which is fine for a Diesel, not for a gas engine.
Boost pressures over 5 PSI WILL destroy your engine with stock pistons, you need forged pistons to run boost
at the levels you are running now, even with then extra fuel, the stock pistons cannot take the heat and pressure
for long, they will fail by burning thru.
As far as the boost building quickly after 10 PSI, it's the volume of the exhaust that gets to the right volume
and really starts to drive the big turbine blade and boost jumps. This turbo in a big Diesel engine would sing
at idle and would be boosting 10 PSI at 1000 RPM and easily hit peak PSI before the engine was at 2000 RPM.
Most big Diesel's are governed to less than 2100 RPM, and the power band is a fairly narrow about 1300 to 1900
and on some occasions 1200 to 2000 RPM, CATs are known to torque well at 1200 and are governed to about
1900 RPM this does vary from engine to engine and some drivers bump the RPM and fuel settings, but in general
terms this is accurate.
You really need to go wrecking yard hopping and find a turbo from a 1.6 to 1.8 or even 2.0 engine, I like the
Subaru turbo from the mid 80's boosts about 7 PSI and those engines didn't get an intercooler, tho I would
recommend one for engine life.
Wild