Some of you asked for a better explanation about restriction in a turbo exhaust, so here 'goes. To sum it all up, on a turbo car, the tighter and more restricted the exhaust housing of the turbo is, the faster you're going to spool your turbo... because the restricted gasses escape through the exhaust housing with more velocity (much like the garden hose description quoted above)... but with this restriction comes the downside. Less exhaust volume will be able to fit through that turbine housing once the turbo is spooled and starts squishing more air through the intake.

This is where the wastegate comes into play. The wastegate is actuated BEFORE the exhaust wheel in the exhaust housing. When it opens, whatever the diameter of your wastegate is gets added to your exhaust piping. Effectively increasing the capacity of the exhaust provided that everything downstream is large enough in diameter to handle it the extra flow. The main reason it does this is to prevent over-spooling the turbo. Once the turbo reaches its efficiency, it doesn't need to flow all the extra gasses through the turbine wheel, so the wastegate allows you to route the exhaust around the turbo... if it can't route enough exhaust around the turbo (restricted wastegate) then too much exhaust gas will be forced by the exhaust wheel and BOOST CREEP will occur as your turbo over-spools.

One way to prevent boost creep is to port the wastegate housing (if you have an internally gated turbo) or to replace the wastegate with a bigger unit. If that still doesn't work, then the problem is likely going to be a restriction in the exhaust downstream from the wastegate. Many overboosted car owners prefer to vent their wastegate dumps to the atmosphere. Not only is it illegal to bypass your catalytic converter, but it's loud as hell, gets your engine bay filthy, and clogs up your K&N like nobody's business; however, it nearly eliminates boost creep. It's a cheap and easy solution that fixes boost creep on a race car.

The best solution to boost creep is to route your wastegate dump past the catalytic converter and back into the exhaust. It will be a custom setup. Nobody makes this. Make it look clean and you'll pass emissions because they don't run your car hard enough to open the wastegate when doing emissions testing. You have to reach full boost for the wastegate to open, and since the exhaust runs through the cat until the wastegate opens, it all gets "cleaned" before it reaches the sniffer. With this setup, the wastegate will also be much quieter because it still runs through the muffler, and you won't trash your engine bay with black caustic funk.

Putting a separate catalytic converter on your wastegate dump is a stupid idea because you'll never get it hot enough to "light off" and start converting the carbon monoxide... so don't get any crazy ideas and create unnecessary exhaust restrictions.

Once compressed air comes into the mix with an engine, exhaust tuning has much less to do with making power. So what if you can milk another 3-5 hp out of a car with a tuned exhaust... the benefits of making an engine sustain an insane final compression ratio (boosted air x compression in the combustion chamber) has much more affect on making power if you can just get rid of the extra gasses it produces. Bigger is almost always better on a turbo setup. The only place where it isn't good is on the exhaust wheel where too big can = no chance of spooling your oversized turbo any time this year.