600 MHz limit is bound in the kernel. I extended that limit to 805 (iirc) just for the fun once, but as it was still not enough to play 720p highprofile, that change was not included. And having in mind that DSP stock speed is 430 MHz, clocking it to 805MHz for daily usage is not something I will include support for in the kernel.

The C64x DSP clock up to 1GHz, but not in 3430 with its 65nm technology. 3630 is a different beer, but i think n9/50 owners can confirm that even at 870 (iirc that is DSP freq in n9/50) they cannot play 720p H264 medium/high profile videos. So it is pointless to increase the speed more than 600.