The 5VZ does use metal plenumn gaskets, so technically you should be ok reusuing them, I've reused metal throttle body ones with no trouble when I forgot to order them. Your mileage does seem low-mine has a cylinder with low compression due to a failed fuel injector screwing things up in it and I still get around 20mpg or better. You may look at the pattern of your 02 sensor and fuel trim if you have access to a scanner. My Tundras O2 sensors are shot and it's made a difference in mileage, also make sure your MAF is clean (hose the two wires inside with brake clean and then gently blow off with air). Hope this helps.