Incorrect backpressure on the exhaust (as the restriction of the catalist, although not a large restriction, has been removed - as well as the catback combined) therefore I'd think a tune would be necessary to help with boost spike etc as you mentioned.

not worth taking a chance on really for the cost of a Verso or ECUtec.

Might as well go Turbo Back and tune and do it properly.

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Arguably my Automobile is Awash with Alliteration - Subaru Sti S202