You don't say which model GT4, only the 205 has a dump valve as standard.
Induction kit:
Pros: Nice sucky noise if you like such things
Cons: Draws warm air from engine bay rather than cold air from outside. Filter surface area smaller than panel filter for std. airbox.
A few years back we were running some diagnostics on a standard 205 on a dyno, and since there was a bit of time spare we had a little playabout. First, we removed the upper airbox section and left the pipe open where an induction kit would fit. This showed about a 10BHP increase - the bonnet was open though so drawing cold air from the dyno fans. We then replaced the airbox lid but without the standard filter. The resulting line laid straight over the top of the +10BHP line. Conclusive proof that an induction kit is no better than the stock setup assuming the standard panel filter is changed for a decent aftermarket one.
Tubular exhaust manifold:
Pros: Shiney. Possible benefits from matched tuned lengths.
Cons: Prone to cracking. Have a large surface area to conduct heat from exhaust to engine bay, thereby both reducing turbo efficiency and increasing engine bay temperatures.
The heat issue can be reduced by lagging, but this will increase the cracking problem. Ceramic coating the INSIDE should reduce all issues.
In practice, unless you spend more than the cars value on a manifold, it will probably suffer all the above issues and probably won't have equal tuned lengths anyway.
Boost controller:
Running increased boost will probably shorten the engines life, especially if it's old and worn. Generally 1.1 bar is reasonably safe as long as it's not a 185 top mount air to air version, however a change to a front mount intercooler will pay dividends both in lifespan and power increase as the charge air will be much cooler.
Dump valve:
The 205 has one as standard, but it's a recirculating one back into the airbox. If you like the noise, it's easy to just take it off the airbox and screw a plate on the airbox in it's place.
There are 185's and 165's out there with atmospheric dump valves, but they do cause issues as they will screw up the mixture while open - especially as many open a little at idle. Using a recirculating one back into the intake pipe after the AFM will get round this problem but you'll lose the noise.
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If at first you don't suck seed, try drier grain.