I will try to give a really definitive answer here, because it’s an important subject…..
Is it the same compressor? Yes and no! The elements of the compressor are the same; the rectifiers, the attack and release circuits and the optical elements, yes, but they are connected in a different way.
The P10 was designed as a 2-channel device where each channel could be used independently. When the ‘LINK’ button is pressed, the audio from the lower channel is routed up to a parallel compressor in the upper channel where it shares a common attack and release circuit with the top channel, but the audio routes through a separate optical network…… so it’s a true stereo compressor, BUT the centre image relies on the accuracy of the optical cells.
By contrast, the P38 and P38EX convert the audio into ‘sum and difference’ mode and then compress both through a stereo compressor similar to channel 1 (stereo mode) of the P10. After compression, the audio is ‘de-matrixed’ back into left and right audio. This arrangement gives technically perfect image centering no matter what happens anywhere in the audio. If there is a slight imbalance in the compressor circuit, the effect will be to modify the width of the image, NOT any left/right shift.
I consider the P38 and P38EX far superior for mastering; not only is there a perfectly stable image, but also there is control of width and balance. The compressor is easy to use, without having to set up individual in/out gains and there are the other functions (sidechain and partial compression) on the EX.
There’s nothing wrong with using the P10 as a stereo compressor…. it’s very good! But only if you don’t have a P38!!