XV273K Can I use a DisplayPort splitter and only one DP out slot on my GPU to activate 144Hz 4K mode

Yep, plug two DP cables into the back of the GPU and monitor, enable the 4k 144 Hz mode in the monitor's OSD, then select 2160p 144Hz in Windows. Without setting the monitor OSD first the computer will see two different displays I believe.

You probably can't, your gpu can't go above 600mhz on DP 1.2 , even when DP 1.2 itself can go to about 666mhz at the very least since that's what dp 1.2 1440p 170hz monitors have, since GPU can't use more than 600mhz there's no warranties that it will be able to do 2x 600 , on top of that, the default 4k 144hz should be at around 1244 mhz or more, and even if you will try to reduce the MHz by manual OC while keeping the 144 Hz you still will be above 1200Mhz , you can try atikmdag patcher though to see if it will unlock the ability to go beyond the gpu's pixel clock limit, I haven't tried it so I don't know if it will help or no but basically yeah no chance for 4k 144 with R9 290x~390x even with dual dp 1.2 cables without modding the driver in some way, you can Google and can find a few disappointed users with R9 2xx 3xx which were unable to run certain 1440p 170hz monitors at 170hz under DP 1.2, simply because these GPUs are locked to 600mhz even when monitor itself can go to 666 etc