It was shown once on the forum that V4 and Mobile can be easily overclocked from 800MHz to 1000MHz. DRAM and L2 cache clocks go up accordingly. All you need to do is to solder 2.2 KOhm to 2.7 KOhm resistor between two pads of serial interface as shown. The resistor goes between +3.3V ( square pad) and TX line of the serial port.
I have overclocked all of mine . I do not see any instability or heat problems. I have looked up DRAM chip and it is at the rated clock frequency.
If resistor is soldered properly, you should see 996.14 BogoMIPS when you boot your Pogo V4 next time meaning that clock is 1GHz. Check it with dmesg|grep MIPS.
I know this is not much of a performance boost but it may help when CPU load is really high.
Overclocking to 1.2GHz is probably also possible but seems to be too involving ( need to put multiple pull up / pull down resistors).
Edit: I have corrected resistor value to reflect what was actually placed on board. Any value in 2.2K to 2.7K range will work. Smaller values may cause serial port problems . Higher values may not pull up the pin high enough during boot time SAR check .