There isn't one marine injector. You need to know the size before we can tell you the approximate rwhp gain.
The spray angle is different because marine engines use different pistons than our trucks. The pistons in a marine application have a wider bowl, hence the wider spray pattern. In our trucks, the wider spray angle cam actually make a bit more power, due to the fuel being spread out more, and burning better. This was the case in a 2nd Gen 12 Valve at least, but I'm sure the same principle applies to our trucks. The main disadvantage is smoke. With a marine injector, your truck will smoke more, but you can keep it under control.
If you keep your foot out of it, you shouldn't see much of a change in mileage at all, unless your injectors are in bad shape. If they are, you'll more than likely see an increase in mileage, as long as the marine injectors are set up right.