An engine is an air pump, the more efficiently it can draw in and vacate air, the more power it will produce.

That said, a good, well thought-out I/H/E combo WILL give you between 10 and 15 HP more[edit:: accented over the stock power curve], and you can squeeze even more out with detailed tuning. Also, with that new-found efficiency, you can actually get better gas mileage around town driving "reasonably."

As far as being worth the cost for the power and time, that's between you and your wallet, but you'd be amazed how much power you can extract on stock internals, stock cams, and no machine work. If you find yourself still wanting more, then you can punch out the bore, balance the internals, port and polish the head, slap some cams in, and/or go the forced induction route.