Over on duramaxforum.com, a lot of those guys like the S&B Dry cold air intakes. Mine is still stock and will be replacing the the air filter with a new stock one soon. They claim the stock intake on the duramax is good for around 500hp and 550hp if you do the air intake mod(remove front plastic part from air box).
And as stated, duramax owners remove the 'engine cover' and plug it to get a loader turbo with a deeper tone out the exhuast. I removed mine and plugged it due to the rubber boot cracking. Didn't really make a difference on my truck but its all stock.