is it worth modifying my probe gt?

hi all
i have a 91 probe gt turbo..
i wnat to know if it is worth to modify and how much gains am i lookin at..
the car has 156k and a auto tranny.. the engine seems sound.. runs good im making 6psi of boost as of now b/c my exhaust mainflod gasket is blown.. so the turbo response is slow..
as of now i have a 2.25 cat back and a cone filter on the car.. doesn anyone know how much this will improve my 1/4 e.t.? and also how much boost can i make when i fix my exhaust mainfold gasket?
thanks
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