It's my first time hearing of project h.
Looks like just a standard O2 PCB inside. They changed the NJM2068 gain stage op amp for the popular and more-expensive FET-input OPA2134. OPA2134 has several dB higher (i.e. worse) noise. Even if the distortion performance were better at the gain used (seems not), the output stage is the "bottleneck" anyway. Several options for the gain stage were measured by the designer for use in the O2. Check the blog.
I don't see any other changes to the O2 electronics, but I didn't look that hard.
You can always change things, not necessarily for the better. And if you do change for the better, it very well might not sound perceptibly better.