I have been shooting the 156 gr Oryx and 155 gr Mega bullet in my 264 WM lately using Hodgdon's data for the 160 gr Hornady and Retumbo. I'm getting a bit more speed from both of them than they did with the 160's which would be expected with a slightly lighter bullet, my gun has a 24" barrel but speed seems just fine. I would start low with data for the 160 gr Hornady and work up slowly using a chronograph and of course some common sense
I thought about trying the 160 gr Matrix but may opt for the 150 gr version instead. I hope you keep us up to date on how things work out for you.
FWIW, Magnum and Retumbo haven't stood out yet with the Mega bullet so I also plan to try IMR 7828 and IMR 7977. Retumbo and the 156 gr Oryx shoots well so that load is done.