All of those mods plus a K&N are as good as you can do on these cars. If you go to techtonics website they actually show that a modded airbox yields more power then an open element (cone filter) style.
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I saw that chart. Since the gain is proportional across the RPM range, I think it's due to temperature rather than to any differences in restriction. There was probably no heat shielding or CAI done for the cone.