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As monkgti says, aftermarket intakes don't deliver a lot of gains but they are an important link in the chain and contribute efficiency to the whole package of performance mods.
Even if very expensive, there's no harm in spending it if you've got it available to spend - People spend much more on sound systems for example. If you enjoy it, that's all that matters. So when am I spending £5k on some carbon Dymag wheels?