After doing a bunch of mods (including different clocks) on my LKS DAC, the type of clock used for the ESS Sabre chips is quite important and will drastically change the character and sound quality of the DAC.
I'm not sure what that femto clock does to the Oppo, but it is likely much better than the really cheap stock oscillator. Keep in mind that Sabre dac uses a global clock at 100 Mhz to do it's timing. The Oppo does not have two TCXO type clocks for each sample rate multiplier (i.e. 44.1/88.2/172.4 vs 48/96/192).
You should also realize that the Oppomod Femto clock upgrade will upgrade the 100Mhz Sabre DAC clock as well as the DSP/digital main cpu clock which will affect HDMI output timing. This will improve things across the board for the Oppo unit.
If this were a normal dac using a non-Sabre DAC chip and had two low cost clocks, I would say that an upgraded clock may still help but I would agree with erik's comment on looking elsewhere first.
