If you’ve pre set your ACC Alt to say 3000’ then setting STD early will mean that the aircraft will now accelerate at FL030 which will be early or late depending on the QNH (unless of course it is 1013)
Not true, below the transition alt. entered in the perf. page the FMGS will reference altitude regardless of what is set in the baro ref. window. You can set STD whenever you like, as long as SRS is still engaged (ie. you haven't pulled open climb) the aircraft will accelerate at the correct time.