Re: 6.5mm twist rate
The more conservative estimates have you shooting the fastest twist you might possibly need: the longest bullet in the densest air (0 deg F, death valley, etc.).
The negatives of spinning too fast are small but real: increased spin drift, more barrel resistance engraving and spinning the bullet, possible lower BC due to rifling marks at a larger angle to flight direction, bullet tip lags a bit more aligning with direction of travel, etc.
Next time I buy a new 6.5 barrel I intend to carefully consider the altitudes and air densities I intend to use it at and buy the slowest twist needed to stabilize my preferred 140 grain match bullets. I can always shoot shorter, lighter bullets if I need to make a trip to lower elevations/higher air densities.
Just to give you an idea on how much you can get away with at higher elevations. My home range is at about 6400 ft elevation. I can stabilize a 69 grain match bullet from a 1 in 12" twist .223 Remington. I can also stabilize several other bullets that recommend 1 in 9" and 1 in 10" twist rates. At this elevation, a 1 in 10" twist will stabilize the 140 AMAX in 6.5, and a 1 in 9" twist will stabilize the 140 grain Berger VLD.