Yay, I have 2556x1440@60, thanks much !
Code: 5.10.35
# DISPLAY=:0 xrandr
Screen 0: minimum 320 x 200, current 2560 x 1440, maximum 4096 x 4096
HDMI-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 597mm x 336mm
2560x1440 59.95*+
2560x1080 60.00 59.94
1920x1080 60.00 50.00 59.94
1600x1200 60.00
1680x1050 59.88
1280x1024 75.02 60.02
1280x800 59.91
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.03 60.00
800x600 75.00 60.32
640x480 75.00 There are still a couple of additional modes exposed by 4.4 with my 144Hz Dell - most of them probably won't be missed, except for the 144Hz one 
Code: 4.4rk
# DISPLAY=:0 xrandr
Screen 0: minimum 320 x 200, current 2560 x 1440, maximum 8192 x 8192
HDMI-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
2560x1440 59.95 + 143.91*
2560x1080 60.00 59.94
1920x1200 59.95
1920x1080 60.00 50.00 59.94
1920x1080i 60.00 50.00 59.94
1600x1200 60.00
1680x1050 59.88
1280x1024 75.02 60.02
1280x800 59.91
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.03 60.00
800x600 75.00 60.32
720x576 50.00
720x576i 50.00
720x480 60.00 59.94
720x480i 60.00 59.94
640x480 75.00 60.00 59.94
720x400 70.08