It is physically impossible to run lower boost pressure than the spring pressure of the wastegate spring, under WOT of course. The reason why is fairly simple and straight forward. Without a WG, your turbo would continue to generate more and more boost until it blows up. The WG spring is sized to withstand air pressure up to a certain point and then give way. Under WOT, you will have a continuous amount of increasing exhaust flow to the WG, which will eventually reach the spring pressure's "give" point to minimize boost. In order for it to run less boost than the spring rate of the WG spring, the spring itself would have to magically change it's composition so that it would resist airflow less (as in less spring coils or thinner coil size for example). Under WOT you have a constant increasing amount of exhaust flow/pressure, and it is physically impossible for it to decrease. I don't know if what I said makes any sense, but the fact of the matter is it is a physical impossibility to run less boost than the WG spring.
The problem is if you go with the weakest spring possible, the spring itself will be that much less resistant to cracking open too early. This can cause problems with slow boost response and inconsistant boost levels. You want your wastegate to do as much of the work possible, and even if you have a boost controller. The reason why is the WG flow is a fairly large volume of air, which is going to require a fairly large area of flow. Boost controllers are simply too small to be doing a large majority of the work. Using an extremely low spring pressure and trying to have the boost controller take care of the rest can lead to other issues. In the case of EBC's, it can actually lead to solenoid failure sometimes as the solenoids will be working that much harder. Also, you can still see some issues like boost creep if using a really weak spring as a weaker spring will require much less exhaust pressure to crack it open. That means you need to get a spring for the lowest boost setting you plan on running.
Also, good EBC's can actually force the wastegate to remain closed 100% until the preset boost level is reached. The end result? You get maximum boost response. Some complain that EBC's are inconsistant, which is not necessarily true. There are plenty of MBC's and EBC's out there that are good quality and consistant. Generally speaking, EBC's are going to be more consistant than MBC's. Some newer EBC's have processors that continously monitor boost pressure and constantly self adjust to compensate for changes in boost level as a result of temperature/elevation changes. You don't get that with an MBC.
Well, 930's were pretty well built from the factory, however that was 30yrs ago. There have been a TON of automotive advances in technology since then, so there is far more advanced equipment and better ways to do things today.
Just FYI so people don't get confused. The boost creep issue people are seeing with Tial's on B&B headers has nothing to do with a fault of Tial wastegates. Tial's are about as high quality as wastegates get. I don't know much about the B&B 930 headers, however from a common sense standpoint, there are alot of issues when designing headers that can cause boost creep or other wastegate/boost control related issues. Some of these are too sharp (such as a 90 deg. bend) or too short of a wastegate runner from the manifold, too small of a diameter for the runner, etc. It's more likely there is some sort of flaw with the B&B design that is causing the inconsistant boost levels. Judging by the pics I've seen, the WG flange area on their headers has nearly a 90 degree turn directly after the flange, which can cause turbulence of exhaust flow in this area. Other than that, I can't really see any other potential issues judging by the pics I've seen. I apologize if I'm reiterating the obvious. I'm not sure if the actual reason is well known and if there is some modification everyone does to correct it. I'm a bit out of date on my 930 tuning info.
Running WG spring pressure as the only method of boost control works perfectly fine. However, seeing as you still have some pressure reaching the WG valve before you reach the WG spring pressure, it is still a possibility that the valve can "crack" open just a bit (yes even on a new WG). With an EBC you can completely eliminate any of this which will result in quicker boost response. In some cases the difference may be minimal, though it is still an improvement regardless and it promotes better consistancy and stability as you have more than one force holding the WG closed.
Generally, from what I've seen it appears like most 930 guys or 911 Turbo guys in general run fairly low boost pressures. When you get into much higher boost levels, alot of things like WG spring pressure and boost controllers become that much more important. Running a low spring rate limits the max amount of boost pressure you can run. Running say 2.3bar of boost for some dyno queen runs (hypothetically speaking) will require a stiffer spring like maybe a ~1.2-1.4bar spring and a boost controller able to withstand that much pressure (some EBC's can't handle over roughly 2 bar very well, generally a result of the solenoid). I don't think too many people around here will have to worry about this aspect any time soon though.
Sorry for the novel!