Is there any downside to using a 7 twist vs 8 twist if I decide to go with a lighter bullet to achieve more velocity?
According to some ballisticians, yes, although I'm pretty hard pressed to remember why. Something about spinning them beyond their intended design makes them not fly as well.
For what it's worth, light/fast bullets need less twist, not more. The 6.5x47 has a tiny bit less case capacity than it's other short action 6.5 counterparts so you can't cram scads of powder into the case to push a real heavy bullet to the point that you need extra twist to stabilize it. If you wanted to neck the x47 down to a 6mm, then yes, you would be shooting lighter and faster bullets that possibly would benifit from a 7.5 twist.
All in all though, an 8 twist with a 123 gr. bullet or heavier, is "the" recipe to get your 6.5x47 singing like a steel guitar and heap big coyote medicine. As far as barrel length, anything from 22" to 25" (shorter is very OK) will perform admirably at those ranges. IMO, long barrels in the coyote field, slightly tip the odds away from the hunters favor, in the way of "handling".
If your really wanting to go light for caliber bullets, the 6mm Creedmoor or 6x47 are super solid choices as well as numerous other 6mm hot rods that are more than capable of bringing home the coyote bacon at those ranges. In fact they may actually be better than the 6.5's for that intended purpose.