In a 7mm-08, roughly how much velocity will be lost if going to a 20" tube? Any adverse effects on accuracy?

A guess would be 40-60fps. Shorter barrels are often more accurate. They would normally have less vibration.

nothing to worry about - paul is right, as usual

The 7mm-08 is not "designed" for speed to begin with, so a minor loss of velocity is not particularly significant.

What may be more significant to you is the surprising increase in muzzle blast that lopping only two inches off of your barrel will cause. Not only is the blast louder due to the shorter barrel (and therefore higher propellant gas pressure at the moment the bullet exits the bore), but also because you have moved the muzzle blast two inches closer to your ear.