Are you referring to the Plymouth splash screen (the purple one with the Ubuntu logo)? This is unaffected by GRUB_GFXMODE, and it's quite possible that the kernel simply doesn't know how to set the mode you're asking for.
Are you referring to the Plymouth splash screen (the purple one with the Ubuntu logo)? This is unaffected by GRUB_GFXMODE, and it's quite possible that the kernel simply doesn't know how to set the mode you're asking for.