I've done it and it's defintely the case. The PCB sound on a lot of Cave games is pretty muffled, even on the CV-1000 hardware. I really dislike the sound on ESP Ra.De.The conversion carts are also supposed to have better sound then the actual PCB version, though I've never put them side by side to compare.
The PGM2 motherboard is on a whole other level of clarity to the conversions though.