Martinc: I hardly use gut strings at all, but have some in my drawer.
Clarkybass asked me and others were looking for a synthetic core D string that might work with the EP Slap set. I don't have that myself, don't slap, so I just tried to find something that could match tensionwise.
I feel there is always a difference in sound and bow feel going from naked gut to any metal wound strings, even round copper wound gut. I think it's a personal thing to accept that or not.
But the transition from wound A to naked D is less noticeable than the transition from wound D to naked G. So getting a synthetic core D might sound better than the naked gut D, but makes the transition more obvious. Maybe a wound gut core D does the job better, but I'm not familiar with the tension of these strings.
I just once played for a minute on a bass with Garbos and their gut G pizz and the gut G was clearly different. Personally I didn't like that, but a lot of players like that set with the gut G.