If you really want a splitter get one welded into your bell mouth dump pipe. I wouldnt recommmend one of those "gasket splitters" though. Or just stick with your bell mouth setup. Its still a good setup and is at least proven reliable. The difference up the top end would be less then 5kw I reckon with a splittler welded in.

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RS Liberty - Because WRX's are only good for parts...