I suggest you buy and read "Turbomania" by Bob Tomlinson. It is available at
www.cbperformance.com
The stock D-Jet is capable of high boost levels, IF... you supply enough extra fuel. The RRFPR (rising rate fuel pressure regulator) will only get you so far, probably around 5 psi max, maybe less.
Then you need to rely on a 5th injector to go higher with he boost. I think Jeff uses the cold start injector, not really sure about that one. The stock type 4 in a 914 will take 10 psi with no problem if you keep it from running lean, and if you keep the oil from getting too hot. Watch your timing also. Too much advance will lead to pre-detonation, a real no-no while on boost.
I had mine up to 11 psi (without an intercooler) a couple of times and it held up.
I would not recommend that though, anything above 7 psi should be intercooled to be safe.