Marginally or not. The serial protocol is only a logical abstraction layer on top of the Bluetooth protocol. The Bluetooth signaling sets up an over the air 'tube' that you can route your data through, independent of what sort of data it is. Data put into it at one end, will come out at the other end. If this 'tube' cannot be set up, then no content is transmitted; if the 'tube' can be set up, then data can be sent through it without any problems.
At a certain point you may get loss of data due to the 'tube' disconnecting/reconnecting, and of course the number of bits you loose at different baudrates will vary. This is not so much to do with pulse width of the data bits, but by the number of packets that are lost in transit.
UART data is not being sent as a serial bit train, but it is encapsulated in packets that containd data and original baud rate. The Bluetooth transceivers decode the original serial bitstrean and encode it into datapackets, the other side reversing it. All being transparent to the user.