I recently made a blackberry version of the raspberry ganache using double the puree (subbing the extra puree for cream) and it was great- really, really great. I did not notice the white chocolate at all, it almost has a diluting effect on the dark chocolate which I think is welcome when paired with fruit. However, I did use Green & Black’s which has a milder taste than some brands of white chocolate.
I also found that mine set up very thick when left overnight. I was able to use it, but it cemented itself to the cake a bit and was a little difficult to spread. The G&B white chocolate, as well as the dark chocolate that I use, all have higher cocoa butter % than average, so I think that’s part of it.
If you’re determined to sub the white chocolate I would figure out how much fat and sugar are in the white chocolate and then use butter for the cocoa butter and sugar, well, for the sugar.