IMHO you don't need the wssm cartridge. The old .243 Win is a classic and fine round which you should be quite happy with, especially if you already have a Howa. I don't think they chamber any of the WSSM calibers.
I've never heard anyone say a Hornady light magnum load was too much for a bolt action rifle. Normally you wouldn't fire Hornady light magnums or Federal High Energy rounds in a semi-automatic rifle. I think Hornady says the Superformance works in a semi-auto, but.... always check what someone tells you on the internet.