This is my whole config.txt:

[pi4]
kernel=9pi4
arm_64bit=1
[pi3]
kernel=9pi3
arm_64bit=1
[all]
gpu_mem=16
core_freq=250
enable_uart=1
boot_delay=1


hdmi_group=2
hdmi_mode=87
hdmi_cvt=2560 1440 60 3 0 0 1
max_framebuffer_width=2560
max_framebuffer_height=1440
hdmi_pixel_freq_limit=250000000

It gives me a 2560x1440 screen with no black borders. That's on a Raspberry Pi 4.

That is with 9Front.

Hope that helps.

Garry