I've been answering a lot of question lately about DPs without cats, fuel cut, boost creep, etc. so I thought I'd put a stickey in place. The following is a simplistic explanation. Feel free to respond and correct any errors I've introduced, or provide further clarification. This is to be an instructional stickey, so more information is always better.
Although some EMS claim they can take care of boost creep, in reality, they are adjusting the actuation speed of the wastegate, which actually does provide some protection against boost creep, but this is a "tuning" type fix, and can't fully protect against the "physical" restriction of the wastegate size.
To the explanation (greatly simplified):
Exhaust flow powers the turbine side of the turbo, spooling up the impeller and compressing your intake air to your manifold.
To control boost, you regulate the flow of this exhaust by bypassing exhaust around the turbo once it achieves set pressure. The wastegate is the exhaust bypass around the turbo. As you close the wastegate, this directs more flow through the turbo, increasing boost. As boost rises to set pressure, the wastegate opens to allow exhaust to bypass around the turbo, slowing the turbo down and lowering/maintaining peak boost pressure. In reality, the wastegate is merely a bypass valve though, not a true pressure contol system.
On the stock system, you have backpressure in your exhaust line from the cat, resonator, muffler, and piping sizes. This works to keep the exhaust flow through the turbo to a rate that can be managed in conjunction with the wastegate.
Once you remove these items and free up exhaust flow (reduce backpressure), your turbo now has more exhaust flowing through it at peak throttle than it did before. The wastegate is sized with the stock backpressure in mind. It now can't flow enough exhaust at peak pressure to slow the turbo down, leading to a slow buildup in peak boost pressure, otherwise known as boost creep. This ultimately leads to an overboost condition, usually exhibited by a CEL and fuel cut (your car suddenly lurches and dies or cuts out and back in at full throttle.....a horrible experience believe me!!!!)
The only way around this is to
1. enlarge the wastegate hole (porting the wastegate and some shops call this porting the turbo..so be clear what you are asking/getting)
2. increase the exhaust backpressure (high flow cat or keep the stock catback with a catless DP)
3 or use an external wastegate.
Of the 3, the external WG is probably the best as you can buy one that controls boost in both directions, unlike the internal wastegate the stock turbos come with. Plus the sound it makes is awesome! Of course, this will mean removing the internal wastegate, so this application will usually be found on a larger turbo upgrade. Another good question to ask if considering a larger turbo...does it have an internal wastegate. If you are going external, of course you don't want an internal wastegate.
Personally, I've always erred on the side of caution and used a DP with a HF cat. No boost creep to date with this setup (on my STi and Legacy). Prior to the HF cat though, I did experience boost creep at a track day once. It is not something I enjoyed as I thought my STi had blown it's engine. Luckily for me, it was just overboost followed by fuel cut.
I have not personally verified using a catless DP with the stock catback exhaust to ensure creep does not occur, but it makes sense so I suspect it will work the same as a catless DP. I prefer the HF cat route though, as it is still emissions friendly and precludes having to swap the stock DP back in to pass emissions. The HP loss is negligable so I consider it the best of both worlds.
I've put in a pic (thanks to JSC Speed) of an internal shot of the turbo showing the wastegate. What you can see in the lower right corner of the turbo is the back of the wastegate valve. In shape, it is basically similar to what you would find as a lid on a garbage can, and this pic would be the top of the lid with the lid hinge off to the right.