The little Topping is fine. It's performance is excellent for the money and it outperforms an awful lot of other inexpensive (and expensive) D/A converters.
The inbuilt D/A converters in the 1300 have the benefit of being slaved to the internal clocks and are more than good enough for RBCD. In fact there are 4 PCM-56 selection K D/A converters arranged as bit shifting 18 bit 8 times oversampling. S/N is specified at 112dB and other parameters are truly excellent.
As such, I don't believe there will be room for improvement with CD sources from that unit.
As the Topping is pretty cheap, you can always try it I guess, but unless you are going to feed it other digital sources, it is, in my opinion a waste of money for RBCD.