Yes, if you are doing a "like mode" bridge (IMBE to IMBE or AMBE to AMBE) then you never have to go through an analog conversion. But if the modes are different, there is no choice.
Now, the following is to all watching this thread:
There is a problem with YSF (it is a good kind of problem to have); the reflector accepts both YSFN (narrow) and YSFW (wide) transmissions. When MB gets these streams, it does know the data type of the audio but it only has one "transmit" port that sends that stream to the other partner. So, in the case of:
YSFW <--> P25 the ports should point at each other and no conversion needs to take place
YSFN <--> P25 the audio must be converted (transcoded) and needs to go through AB.
I am proposing adding a new transmit port definition to the DVSwitch.ini for YSFN and YSFW. You would then be able to point the Wide ports at each other and the Narrow ports to a transcoder. Also, note that this is only needed for YSFN -> P25 because a Fusion radio even in narrow mode can receive Wide transmissions, so the P25 -> YSFW will decode perfectly and does not need to be transcoded for the conversation to succeed.
So TXPort would be deprecated and TXPortN and TXPortW would be created for the [YSF] stanza
P25 -> YSFW would point the P25 TXPort at the YSF RXPort
YSFW -> P25 would point the YSF TXPortW at the P25 RXPort
YSFN -> P25 would point the YSF TXPortN at an AB transcoder (which would transcode AMBE to IMBE) and send it back to MB on the P25 RXPort