Don't do just an intake. Do the whole shebang: intake, downpipes, catback, etc. If you want power. For looks or sound, you certainly have options. As amply stated before, the simplest and most effective intake-only mod is the dropin K&N. The rest depends on your needs, style, and budget.
Currently these are the vendors that offer intakes for the SHO: K&N, Airaid, LME, MDesign.
A mod that does seem to help at the track is a "cold air" tube from the snorkel down to the front "fog" light area. bamsho & brucelinc have made this work.