Hi clive,
From Airbus FCTM SI 060 P3/4
If a RA is generated: • The flight crew must always follow the TCAS RA orders in the correct direction, even: ‐
If the TCAS RA orders are in contradiction with the ATC instructions ‐
At the maximum ceiling altitude with CLIMB, CLIMB or INCREASE CLIMB, INCREASE CLIMB TCAS RA orders ‐
If it results in crossing the altitude of the intruder.
I think you'll find it's cheaper to nibble into the buffet margin rather than hit the other aircraft.