Welcome in the forum, Gareth.
No, the choice of using internal buffer and interrupt cannot be reverted a t run-time.
You may of course provide your own algorithm which puts character-by-character into the transmitter register and waits for fifo getting empty again.
You can configure the interrupt priorities (in the design-wide resources). AFAIK its not possible to change the buffer size and the interrupt usage on run-time.
Ah. Thanks for the info, appreciated.