Barrel length and cartridge size?

[6.5grendel]

Would the advantages of higher velocity from a higher case capacity cartridge vs a small case capacity cartridge be less pronounced in a short barrel? For example would the velocity difference between a 6 Creedmoor and 6GT be less pronounced with an 18" barrel than it would be in a 26" barrel? Reason for asking is I'm putting together a shorter barreled rifle that will always be running a suppressor and if I won't be gaining much with going to the bigger cartridge I would rather stick with a smaller cartridge and save the powder.