noob needs help with mileage - only 10 MPG!

More than 20% is considered suspect, and certainly 25 and greater is not that great.

BUT does it use oil? Did the shop say WHERE it is leaking, IE valves, rings, what?
What did they say about the "rough idle?" Did anyone ever set the valves? They should have been able to get a clue with the leakdown test.

In my opinion, this is probably NOT bad enough to explain the very poor mileage, and it seems to me you are asking this question before you have found out IF you've managed to improve it.

I think the answer to an overhaul is based on you: Your expectations, just how much you intend to drive it, and how long you may be keeping the vehicle.