downshift.
having your peak torque above 3500 rpm only hurts you between 0 and 15 miles an hour or so. after that, use your gearing to keep the rpms where they need to be and it will perform the same as it would if you had a much lower powerband. highway driving is actually where an engine like the 3.0 shines (think about it, its a sedan engine...and where do sedans spend all their time?)...you just need to not be afraid to downshift and kick the rpms up for hills, passing, merging, ect.
on top of that, bringing the powerband down will likely lower your peak power output...meaning the engine will ultimately be slower than it is now. the 3.0 already puts out a very good power curve for what it is....it makes almost 85% of the engines peak torque at 2,000 rpm....you probably wont be able to increase that number much.
im not trying to talk you out of a new cam...i just want to make sure you understand that i dont think it will give you the result your looking for.