You should always check the gap before installing plugs. They are factory gapped platinums when you get them at the dealer but it only takes a small tap or drop to close the gap up. I dropped a plug once before putting it in an astro van and believe me, it was running on 5-6 cylinders until I found out which plug it was. Weird thing was the van felt more powerful than ever. Maybe beav could explain why. If you've ever even seen under the hood on one of these you'd learn the lesson I did. Changing the svx plugs was easy in comparison. I agree you shouldn't run cheap plugs but I have never tried the bosch +4, I just go with the factory platinums.
I drive through VA and WVA right over the hills about 6 times a year and I still manage 26mpg through there doing a steady 75-80mph but that is with only 1 or 2 stops during a tank. Now if thats 45-70mph driving in hills, you're gonna spend alot of time with the TC unlocked and doing alot of downshifting and that will definitely drop the mileage down. I try to always go fast enough to keep the engine above 2600 rpm when I am coming up to a hill so that the car has enough power to go right up without unlocking the torque converter. If I am only going around 60-70 I won't make it up the big hills without the TC unlocking and possibly a downshift. On an 800 mile trip it makes a difference. I have never hit 27 mpg but my wife has once on the same trip driving my car so I am on a never ending quest to beat that. I still don't know how some people here get 30mpg.
Excessive black soot in the tailpipe indicates you are running a rich mixture. You're burning more gas than you should be. This could be due to a number of different things. Some black soot is normal. You can pull out your spark plugs and check their condition to see how your engine is running. They have pictures in the back of most chiltons manuals that will show you how to read a plug.