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will a custom 2.5" downpipe de-cat flagg up an emmisions light? will this just flash or will it cut my engine eventioally? is there any alternatives instead of getting a stage 2 (right away)

Any de-cat will flag the emissions light

Whoever is doing your re-map can "map out" the emissions light so the flag tolerance is higher, thus not coming on unless there is a huge problem (So I was told by my local revo remapper)

true, they can read it out so the light stays off
but it must be done because you will def. get a warning light!

Otherwise some have spaced the sensor further out, but as the engine uses this flow / heat info to work with the ongoing tune (i believe), you'd want to space it just right.