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Good guide over on cliotrophy and it covers pretty much all of your questions...
In short as far as I understand, Acoustic valve is there to restrict noise to meet EU regs (although not sure if this is actually required in the UK) and the benefit of removing is supposedly increased airflow and more noise from an induction perspective... Although actual performance benefits are open to interpretation.
Check out the guide and see what you make of it, procedure is fairly straightforward