Another thing to keep in mind is that hashcat can generally test guesses faster then you can feed them over the various connections, i.e. memory -> memory copy for pipe memory -> pcie -> card for loading to the GPU. Also, in cracking rigs, you often have multiple cards over pcie x1 connections, further lowering bandwidth for those cards. One thing I'd highly recommend is trying it with a rules. This allows the GPU to expand your word guesses out on the GPU itself, turning a single guess into thousands of guesses or more. This bypasses memory bandwidth issues, as it's done on the GPU itself, and also means that a lot of rather trivial checks can happen automatically (try it with a 1 at the end, try it with a 2 at the end, and so forth).