This information comes from an e-mail with a Borg Warner technician (Edit: Garrett retailer) that I can reproduce here if necessary (it'll have to wait until I get home).
Here goes.
Turbocharger oil seals will handle just about any amount of oil pressure you can throw at them. Apparently turbochargers have more trouble with air getting into the oil than oil leaking past the seals. I know this goes against what you've heard. Turbo oil seals are mainly designed to keep intake boost pressure and exhaust pressure out of the oil system. They are not designed to keep oil in, they are designed to keep air out. That's not to say that they don't keep oil in, it's just the way the seals are designed. It is much easier for the seals to hold the oil pressure in, so they actually have a much harder problem to deal with, which is keeping the pressurized intake charge or exhaust gases out of the oil system.
We could use an example. Okay, assume you have a typical turbocharger with an oil feed line with no restrictor. This allows 80-120 psi oil pressure inside the bearing assembly and places 80-120 psi pressure on the oil seals. On the other end of the oil seals you have let's say 20 psi of boost pressure, and for fun let's say we have 20 psi or exhaust pressure. This pressure on the other side of the seal makes the net pressure on the seal 20 psi less. So the seals really see 60-100 psi of oil pressure. This is what they were designed to handle and actually make use of the high oil pressure to keep intake and exhaust gases out.
There's a problem with this situation on smaller turbos. Small turbos typically have smaller oil drain outlets. Improper oil drainage is the true cause of compromised turbo oil seals. There usually is no way to drain oil fast enough from a smaller turbo with it's small outlet to keep up with the flow of oil through the turbo. Once the oil in the drain backs up to the bearing the pressure placed on the turbo oil seals usually increases until the oil seals are compromised.
The problem gets even worse with ball bearing turbos (as compared to journal bearing) where the bearing operation introduces much more air into the oil (frothing) than typical journal bearings. This creates a much quicker overload of the oil drain, causing the backup to happen faster.
Have you ever wondered why a larger turbo oil seal could handle higher oil pressure than a smaller one? It doesn't make any sense at all. It's not that the oil seals are weaker in small turbos. It's that the oil drains are smaller.
Have you ever wondered why ball bearing turbos needed restrictors when a journal bearing in the same application wouldn't? It seems to make sense that a ball bearing has tighter tolerances than a journal bearing, so it would need less oil, but why would it need less pressure? And why would more oil (flow) in the feed line be a problem? The reality is that the ball bearing can pass just as much oil as a journal bearing, it just makes it more frothy due to the tighter tolerances and design. It's this highly frothy oil and eventual oil drain back-up that causes the observed problems with aggressively feeding a ball bearing turbo.
Big turbos usually have big oil drains and can run largely unrestricted lines. Small turbos have small oil drains and usually require a restrictor. It is true that this restrictor lowers the oil pressure seen by the turbo oil seals, but the lower pressure is not the solution to the problem, it is the lower associated flow of oil that solves the problem. Too aggressive of a restrictor can actually cause a different problem. You can lower the oil pressure with a restrictor so far that the oil seals can no longer keep the intake or exhaust gases out of the bearing section and bearing failure can occur. This is the real problem the turbo manufacturer has with their seals and more likely the cause of failure in overly restricted feeds (usually blamed on not enough oil/lubrication).
I know this is going to be controversial so if you need anything clarified, don't hesitate to ask.
.I found this info which suggests it might not be down to the restrictor at all.