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IF you can find a blown s5 turbo you can combine it with your good working s4 coldside. The whole idea is finding one cheap and using yours.If you do a hybrid down the road an s5 is better because of flow+wastegate. The s5 does have a twin scroll sort of.
When I got my turbo upgraded by BNR, I did it with a Stage 4 series 4 turbo. Brian suggested that to me because I was looking for max power with the expense of a little more lag. If I understood him right, he explained to me he can port the wastegate bigger on the S4 turbos and they could push out more max hp with the expense of a little more lag. The S5 turbos will spool faster on the engine, but make overall less HP then the S4 turbo.
But as Aaroncake said, there's no way you'll get 300whp out of the stock turbo... Possibly if you do some type of AI system as well.