On my Basesquatch I use 5th gear unless it's very flat or downhill. Have tested gas mileage in 250 mile consistent trips and found 5th to be better. I tend to stay between 70-75mph though so maybe at 80 things change. Also browsing through forums I found the focus rs (very similar engine...
1. Yes factory enclosurre you can use
2. No amp present
3. Not sure about wiring but there are a few people on here who have used the existing enclosure.
RagnarKon has a few good videos about this on YouTube. Check it out.