You're correct, it's not 100% totally sealed,
if you peak under the hood once it's almost closed you can tell there's a bit of a gap near the fender.
My theory is there's not very much warm air along that small area once you're moving anyway
If you were really concerned you could probably use some pipe insulation but it's never concerned me.
With the Airaid/Magnafow CatBack/Diablo triple whammy I'm in the 13's
with the help of a few minor supporting mods (180º T-stat, Catch Can, Y-pipe)
...and a set of dedicated Drag Tires to help my poor 3.55's out.