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Unless you plan to have some form of forced induction i see no benefit from having twin 2.5". If its a single 2.25" now then going to twin 2.25" would be plenty for most mods youd do to an alloytec. Rocker upgrades and even a cam would be fine on twin 2.25" along with other supporting mods.
The larger you go in exhaust means you'll lose exhaust back pressure so in a way you'll lose some torque. So only need to go larger if your going to have a greater exhaust volume from rockers, cam or forced induction.
Should just do a full exhaust (extractors, high flow cats and a twin 2.25)