Are your router/switch/hub/connecting devices all Cat 8 capable? If only one of them isn't isn't (and since the standard is really only in use in the highest performance applications within datacentres, this is not just highly likely, it's almost certain that none of them are) then you are wasting your money buying a Cat 8 cable. It's not as if Cat 6 is any slouch, never mind
Cat 7 which can be bought for less than £3 per metre terminated with quality gold plated connectors.
In reality any perceived bottleneck in performance is unlikely to be the patch cable. See below on how to choose a network cable.