BTW a Turbo is a type of supercharger. Just exhaust driven instead of belt driven. Turbo's are probably more efficient than a mechanically (belt/gear) driven supercharger, however turbo's do have more lag and produce very high temperatures.
Remember that when you up the boost a lot with a turbo, the air gets very hot, so you may want an intercooler. Also remember turbo's are generally better for high rpm power rather than low rpm power. The 318ti is absolutely terrible for low end, and unless you want a really peaky powerband (worse than it already is), I would go with a supercharger.
As for dual exhaust, I think it is worthless on almost any vehicle but the largest V6/V8's. A small 4 cyl doesn't need much exhaust flow, even if it is heavily boosted from a turbo or supercharger. If you really want the look, just do what all the factory 4 cyl. dual exhaust cars do, and use a muffler with two outlets. One on either end, so you get the dual exhaust look without adding much weight and cost to the exhaust system. And you will probably have to remove the spare tire like someone else was saying. If you do add a turbo, you could probably do away with the muffler entirely, and just split the exhaust pipe towards the back of the car and make it look like dual exhaust. Turbo's quiet the car a great deal, the Neon SRT-4 doesn't even come with a muffler from the factory (from what I understand). Turbo's absorb heat from the exhaust, which reduces the noise produced because the air expands less rapidly when it leaves the tailpipe. This is why the turbo itself get's so hot.
If you really want fast acceleration with this car, the cheapest (and most reliable) option would probably be to use an inline 6 from one of the other three series models. Then you will get a lot more flexible power band. You will actually have both low and high RPM power. | 
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