Answers 2 of your questions. From:
Project 'nerd gear': taller tranny transplant nets +5.2% MPG - MetroMPG.com
Green line = baseline. Orange = taller geared transmission. Some aero mods in place (no belly pan, but partial grille block, rear wheel skirts, passenger mirror delete)
EDIT: note, that is a fully warmed up car cruising on a nearly perfect, flat road with no other traffic around. In other words: test conditions, not results you could expect from a real world "trip" that used that cruising speed.
They are lightly built cars, but surprisingly durable.