140+ miles a tank sounds pretty good. Just cruising on the highway should net you about 40 mpg, which is exactly 140 miles a tank from topped off to fuel light coming on(3.5 gal).
HID lights with proper projectors cost quite a bit and is the best option.
You can also spend about $50 for HID lights without projectors. Some people say that HID bulbs in halogen housing is bad juju. From what I've seen, the light itself is not bad, but it allegedly blinds oncoming traffic.
There might be conventional halogen bulbs out there that is better than OEM, but not sure about cost and whether it would be worth it.
For long trips, you want to make sure you have a comfy seat and remember to relax your forearms. It's generally been my experience that the OEM seat starts hurting the butt within a few hours of highway cruising.