I used to have an Oppo 93 with Exemplar mods, it sounded very nice for the money. I believe it had the usual stuff like power supply upgrade, nicer opamps, all the regular mods that different companies did with Oppo players over the years. However I think it almost exclusively helped the analog outputs. Because I never noticed an improvement when using the 93 as a transport with external DAC. Granted I only ever tried coaxial digital out and not HDMI but one would think the same applies for both.
That Oppomod power supply is $380 so not the end of the world if you wanted to experiment. I just wouldn't do it expecting major improvements as you might be disappointed.