What is the difference in diameter between the 69mm Bored out throttle body compared to a standard one? And how much will this increase the horsepower?

The factory one is 64mm in diameter while ours is 69mm in diameter. In saying this though the original factory figure is a little misleading, given that there are a series of humps inside the factory throttle body, which brings it closer to 60mm. Gains of around 3 to over 5rwkw are not uncommon, depending on your current mods.