We've spoken about this before. The blocked off pipe is a resonance chamber setup that stops droning etc and has been tuned for the engine. Many people cut this open and run another pipe or whatever, however all this does is increase noise and REDUCE power (proven fact).
There are significant gains to be made by changing your exhaust, however that should include headers and the whole system. Venom XR had his XR6 HP done with the Herrod system which sounds good (even for a s!x) and made a big gain in power and more importantly torque.
Ok not to put down Herrods but I would recommend using workshops closer to home than using interstate shops whenever possible. There have to be at least a few places in Sydney that can do decent exhausts and I believe Herrods is a VIC only operation currently.
However, I was already heading to Victoria for the Ford Forums trip, and it meant I left a day early to get the system installed.
However, NZ XR8 (I think that's right) has a Herrod system on his 5.0l, and he has the Tasman sea to worry about! So you can probably have his system, or parts of it, shipped to you if you're keen for it.
Local Sydney exhaust places that do good work? No problems. Tuffy Mufflers at Woolloomooloo (damn that name) and make sure **** works on your car. I had a system made for my Volvo 850R and it was SUPERB. Looked factory fit but neater. Linished welds, mandrel bends, great looking polished mufflers etc. And very good price compared to some ripoff merchants such as that crowd next to Ryda at Petersham on the main drag (they wanted $2500 for a 2.5 inch system on my Volvo!