The problem with engine fire case is both engines are running so the rate of climb is high and you reach EO ACC quickly. If you level off without securing the engine then you have to continue the ECAM to secure the engine in level flight as putting out fire has top priority, the VFE can be reached and you will have to quickly retract flaps and get back to attend to fire. It can be done, but on your off day you may damage flaps. Even if you did not you gain nothing by doing this jugglary. So take your pick.