First, I don't load for the 6.5x55 but have done so for the .25-06 so the twist rate experiences should be the same.
The 1:9 twist should handle both weight ranges you have mentioned. One nice thing about the 6.5s is the ability to take deer sized game cleanly with lighter bullets that the more common .30 caliber loads. The 160 grain 6.5mm bullet has a sectional density similar to a 220 grain .30 bullet. I suspect that there would be few reasons to "need" something that heavy for the caliber. The 120 grain 6.5mm bullet has the same basic SD as a 165 grain .30 bullet. The 140 equates to the 190 grain .30 bullet.
My personal preference for deer sized game would be to try the 120 to 140 grain stuff in the bullet of choice and see if the rifle has a clear preference. I'd be more than happy with any of them so I'd let the rifle pick.
Lance
