[blackbird]

With how small the stock turbo on the 1.4L is I'd suspect that a conventionally bent (i.e. crushed) 2.5" wouldn't flow significantly better than an aftermarket mandrel bent 2.25" exhaust system, and both are probably overkill for the exhaust flow and power the engine can make so I wouldn't stress too much on what to choose. If you're comfortable with welding, have access to some pre-bent mandrel stock and take pride in doing your own work along with saving some money nothing wrong with that. But if the overall cost is similar nothing wrong with a quality aftermarket system.

 

[blackbird]

Have you had a chance to build any exhaust systems before? Do you plan to remove the cat?

I'm more familiar with the GM 2.0L LNF turbo and the diesels, but looking at the stock and aftermarket downpipes for the 1.4L you'd want to replicate the flex pipe expansion joint otherwise you're going to have problems with the exhaust system moving when the engine rocks on it's mounts that will likely crack something, and hopefully not something on the forward/hot side of the downpipe.

You also need to factor in quality flanges, the exhaust hangers, and O2 sensor bungs. Your welding skills need to be very good to make sure you don't warp the flanges otherwise it might have problems with sealing.

If you don't have a jig and just go about eyeballing it might be challenging. Personally the prices aren't too unreasonable for the aftermarket downpipes and you're getting something that should have been built on a jig and will fit on the car. Perhaps consider picking up a pre-fabbed downpipe and build the cat-back yourself. If you're up for a project and have some spare money to burn a downpipe might be rewarding to build but it could take a lot of time to get perfect fitment.

 

[blackbird]

If that's the case go for it. If you're relocating the cat and trying to retain the pre and post-cat O2 sensors, keep in mind you don't want to move the cat too far back under the car. If the exhaust temp drops it might take longer to warm up and possibly set a code. The programming in the engine controllers can be a little over zealous to make sure everything is running how it was designed.

Also when you're welding on new O2 sensor bungs make sure to keep them between a 10 o'clock and 2 o'clock position on the exhaust when installed. If you angle them downwards, after the engine is shut off and the exhaust system starts to cool gasses can condense and form moisture. You don't want that building up and dripping down into the sensing cell at the tip of the O2 sensor. It can dramatically reduce the service life of wideband oxygen sensors.