I didn't notice before, but actually that vertical backporch value seems too high:
monitor_specs_0 = "15450.00-16050.00, 54-65, 3.19, 4.75, 6.45, 0.826, 0.191, 1.462, 0, 0, 288, 448"
1.462 equals 23 lines (too many I think). I'd use a value slightly above 1.000. Again, you can use Arcade_OSD to find the value that's good enough, you'll notice it's too low when top of the screen starts getting distorted.
That's probably why you're getting such high horizontal frequencies for those otherwise usual modes. Using lower vertical porches allows for creating the same resolution+refresh at a lower hfreq.
Tomorrow I'll post some links about how you can potentially damage your monitor by 'overclocking' it. Always take care!