Can this stm32f0 interrupt routine that activates on every byte received be fast enough for 6Mbps?
The possible speed of any software routine, whether it be an interrupt routine or not, can be easily profiled. The way this is done is to commit one of the GPIO pins of the MCU as an output pin connected to an oscilloscope. (A logic analyzer can work too). In the piece of code that you want to profile you set the GPIO at the start of the routine and clear it at the end of the routine. Run the code and measure the resulting GPIO pulses on the scope. 

Using this technique there is no longer a need to ask questions as to 'is this fast enough' or 'if I change the code how does it affect the timing'.