Most say stock is the best. Some say it's better with a efficiently designed chargepipe and stock airbox. Others say that's not worth the money. Your best bet is just getting an air filter and calling it a day, really. I got Gruppe M intake plus Dinan Chargepipe, and while I did see some improvements, it probably didn't justify the cost. I'm sticking with N/A and I'm not stroking my motor though, so I'm happy with it.
Wait, I mean, I don't have a Gruppe M intake or a Dinan Chargepipe. My signature says so.
Past: Toyota MR-S (Small)|Toyota Solara SLE (Terrible)|AP2 Honda S2000 CR (Rear-Ended By Big Rig)|E92 BMW M3 (Lemon Law Buyback)|Second E92 BMW M3 (Mechanically Problematic, Sold)
Present: 997 Porsche 911 (Perfect)|Mitsubishi Lancer Evolution X GSR (Impractical Daily)
Future (Planned): Subaru WRX STi Hatchback (Practicality Coming Soon)