Exhaust is very unlikely to do anything for mileage if you're driving for mileage. The engine is just not pushing enough exhaust through the muffler to cause much of a restriciton issue.
For intake, you may see an increase if you pull in warm air from inside the engine compartment. This seems to work on some vehicles and not on others.
Check out the 65+mods and 100+tips at the header of Ecomodder. There are plenty of things you can tinker with and not void your warranty. As always, adjusting your driving will always reap the single largest gain for the dollar. Using a
scangauge will further help you train yourself and can actually pay for itsself if you use it.
Not sure what the latest on Bondos aerolid is. He's done a really nice prototype but production opportunity seems to be the brick wall for him. I hope he is seeing some progress in getting it to market.
Please chime in Bondo if you're still with us.
If Bondo is ok with it, we should all contact each pick-up shell manfacturer and ask if they have or are planning to make an aerolid or fastback style shell. Enough feedback and one manufacturer is bound to start at least a limited run. Someone already makes one for the Hummer, of all things, they probably don't sell many of those. The MPG market for pick-up aerolids is undoubtedly much bigger.