Last week also I had changed the timings on BB, but I have solved my problem, and I tried lots of configurations, but I dont know that it will help you.
For my LCD, if I remove the MR flag from the dvimode for bootargs, the output is only black screen, so you can try with MR
Also I have tried different Hz values and finally I found true value on 16@75
And you can change the resolution, not constant 480x272 maybe 800x600 works
Good Luck