exhaust

bulldust,
Whilst I haven't seen this done to a Prado it is of course a tried and tested way to get more power out of an engine. Your first problem may be sourcing extractor/headers, I have a feeling they may not be available for a Prado "off the shelf" due to the lack of market. However I may be wrong and of course you can always have them custom made(Expensive!!!!). Next, remember in a late model vehicle such as yours header/exhaust design tends to be far superior to that of days gone by, so don't expect any huge performance gain.
With improved intake(snorkel/filter) extractors, 2 1/2inch(mandrel bent if the budget stretches that far) High flow cat, straight through muffler and resonator expect around 10-15% more peak power.(remember thats "peak power" not right across the rev range!) A little more can be gained by rewriting the ecu ignition/fuel map to take advantage of these mods, however this is costly and the gains not great, also it may mean a permanent diet of premium fuel.
Also be aware that improved exhaust flow always goes hand in hand with increased noise (may sound great around the back streets but on the open road the constant drone may send you mad!)
Another possible disadvantage is a shift in the power/torque curve(max torque/power higher in the rev range) and there is a chance of some loss in the bottom end torque.
Choose a workshop that can provide before and after dyno testing of your vehicle.

For the cost, they arent worth it...

I would have to agree with truckster on that one.Putting a bigger diameter exhaust may have worked on older cars but these days with owners demanding more economy and performance from their vehicle combined with a higher weight from all the extras and safety features,most engines are operating pretty close to their maximum as it is.To get much more would require a major rethink/rebuild.
Even on older cars, bigger exhausts only improved the performance when the engine was operating at %80-%90 of its maximum output.Anything under that the exhaust was not restrictive.
25 years ago you would use a 5-6 ltr V8 to lump around 1.5t ,now a 3.4 V6 is expected to lump around 2.8t and you still want more!

bulldust,

Something else to think about and maybe add to your christmas wish list; Today I had a job on a 100 series Landcruiser, 4.5 petrol. This vehicle was fitted with a supercharger kit. Info from owner as follows: C.A.P.A powerdyne supercharger kit, Stage 1, 245kw(up from 165) around $6000 complete. Can be upgraded to around 290-300kw!!. Don't quote me on these figures! owners of powered up cars tend to stretch the truth!! Now I can't say they make one for a Prado, but it may be worth checking out if "money is no option". The kit and installation looked very professional with all plumbing, brackets etc looking well made. C.A.P.A are in Adelaide, but sorry, owner didn't have their phone number. However they are online so you can contact them through their website I guess.

www.capa.com.au

Hi Bulldust , don't forget that modern vehicles are built to the lowest possible price, and have to acheive some very low levels of NVH so that they are appealing to joe and mrs joe average. Therefore the induction systems are heavily restricted with noise baffles and resonance chambers to cut induction 'roar'. The exhaust manifolds of modern cars are heaps better but the pipes are often restricted in various manners to cut resonance as well as having restrictive mufflers. For the lowest cost improvement, first fit a washable element high performance air filter (uni-filter / finer filter etc.) this will give better aircleaning and better flow as well as saving a lot of bucks in the long run, then if you do water crossings - fit a snorkel with an un-restricted pipe into the airbox, if not you can fit a cold air intake within the engine bay and open up the inlet to the airbox. You will notice a good diference in power and economy straight away. Then you could get a good exhaust shop to fit a new system using high flow cats and straight through glass pack mufflers, it may not be justified to spend the $600 odd bucks on custom headers. Modern multi valve milti cammed engines have huge volumetric efficiency compared to old guzzlers, and there is a heap of efficiency to be gained at the expense of some increase in noise. Remember efficiency equals power... BradLife is short- but there's always time for a yarda.