The stock gearing is so nice on the 658. As is a relaxed rpm at highway speeds.
Tall enough IMO because some headwinds & mountain passes whiled loaded (touring gear silly) will have you going down to 5th or 4th at times.
1st gear is usually low enough for most situations. I have done very technical two track trail and the 658 works well at idle through those situations.
If you are dog paddling through technical stuff a -1 countershaft sprocket could be advantageous.
Over gearing it will have you going a gear or two lower (than normal) on the passes and you will be doing the same rpm's anyway...
IMO from idle to top speeds the 658 is geared well and gets good fuel mileage doing it.
From my experience cruising from 70-90 kills mpg's (upper 40's). 55 to 70 and the 658 pushes 60 mpg (57-58 for me)