| please take this as a correction to your statement and nothing personal. I like you and hope that this isn't taken the wrong way - I just want to clarify this a good bit. VE and peak achievable boost are not related at all. If this were the case, as other breather mods are performed on the same vehicle (short of turbochargers), the peak boost achievable will continue to lessen. This is not the case, and if anything, it can actually increase the peak boost pressure achievable by the system. I think your statement regarding VE and boost goes back to a statement of misconception by Greg Dupree about the GT2860RS compressor flow maps and why they weren't able to achieve over ~24psi on the vehicles they have put those turbos on. Improving the volumetric efficiency before the turbocharger, as in switching from a single intake to a dual intake, will improve the peak boost the system can achieve. This comes down to the fact that a turbo compressor is only a pressure multiplier: a restrictive filter/piping set feeding the compressor will lower the ambient pressure at the compressor inlet and reduce the peak boost observed at the gauge. Improving VE after the turbo, such as in a camshaft upgrade, will simply mean that you are going to move more air mass per revolution of the motor - this does not have any effect on how much boost a turbo will be able to produce. Say the exhaust manifolds are upgraded - this will improve airflow through the engine (VE), and it will also improve spoolup response of the turbocharger. The turbos will get spooled up quicker than if they were using a stock exhaust manifold..... and this brings me to my final point... :) The major component that depicts how much peak boost you will be able to achieve with any of these turbochargers that are suited for our engines is the turbine and housing selection. The wastegate and its actuation system can play a part, but only if they are not properly chosen. You need an actuator that will provide ample wastegate closing pressure to prohibit exhaust gas bypass inadvertently. There are components you can upgrade in the system that will have an effect on at what RPM you achieve that peak boost, but will not affect the peak boost level itself. There are other elements in the system, like the intake example I gave, that can have an effect on the peak boost observed in a given application, however, the maximum boost that a compressor can produce has nothing to do with the volumetric efficiency of the engine. The compressor does only what the turbine tells it to. If your engine has higher VE than another engine with the same turbo, you will move more air through the turbine which spools the compressor quicker and the effect is only on the rate at which the rotating group accelerates. Variations in turbine selection also play a role in the boost response and can also indirectly affect the peak boost one can achieve. A prime example of that is: look at the dynochart for my Z32. I am using the same compressor wheel and housing as that of the GT2860RS. I am making 1bar of boost by around 3400RPM and I have seen some ~31.5psi of peak boost. My turbine is different from the GT2860RS by a difference of only 1 blade added to mine, same trim, same geometry - just an extra blade. This extra blade is harnessing a bit more energy from the exhaust and causing the rotating group to accelerate in rotation faster, causing the noted improvement in spoolup as compared to the native GT2860RS .64TH. If you take this one step further to inspect the response, compare the results of my setup vs. that of Greg D's or Seb's car as they have the original GT2860RS with the large .86A/R housing. What Seb and Greg are experiencing is the fact that the turbine housing they are using is not allowing the rotating group to harness enough exhaust energy until the engine's airflow (RPM) has passed beyond the region of the compressor's operating range that offers the highest pressure ratio. They could put a stiffer actuator in there or even go as far as to weld the wastegate shut and it would not affect this. Here's an example of what I mean about how the turbine and housing will affect how quickly you can spool up the compressor per engine RPM and how this can affect the peak boost attainable: (not a 100% accurate depiction, but it shows what I mean): RED: GT2860RS w/ .86 A/R turbine, 9-blade
BLUE: GT2860RS w/ .64 A/R turbine, 9-blade
GREEN: GT2860RS w/ 10-blade turbine & .63 A/R housing 
The lines represent how the turbine spools up the compressor at its maximum ability. You can see how the red plot prohibits you from obtaining the peak pressure ratio of the compressor, the blue puts you slightly after peak (I haven't seen or heard of the GT2860RS w/ .64 A/R turbine housing produce 31psi before - the most I've seen is around 26psi), and the green line representing my turbocharger variant. If the turbine can't get enough exhaust energy to spool the compressor fast enough to keep up with the airflow demand, you will not achieve the peak boost available by the compressor.
When it comes down to it, if you are having problems achieving the peak boost that the turbocharger's compressor can offer, you are not properly choosing the components used in the turbocharger's construction. The GT2860RS compressor/housing is an excellent combination for modified VG's covering a rather wide range of setups - what is making the most difference in observed performance and peak boost is the turbine and housing selection. If you use too large of a turbine housing with this turbocharger, you wont generate sufficient shaft speed per engine RPM to obtain the peak boost capacity of the compressor. So, it has nothing directly to do with the VE of the engine, although it could be perceived that way, incorrectly, of course. :)
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