How does that work?
Long time out of the system now, but it used to work based on your original ETA, this was put into the MAESTRO system and your 'slot was assigned' by manual insert, you then take the delay anyway you want, on the ground or in the air, it won't effect the order of the sequence as you are already in it... The biggest problem was then not getting away on time after a ground delay, but still trying to get the original revised landing time, based on how you see the estimate, not how the ATCs and MAESTRO sees your estimate; things like short cuts and increased speed to make your original spot aren't going to work when there is a solid queue in place, it's hard for us to organise an ad-hoc overtaking lane.
I've also encountered a few grumpy sods ex CB who need to get a spin or two on the way to SY after taking a ground delay; **** does happen, slots get missed, go rounds, higher priority push-ins etc, your spot in the queue doesn't change but your landing time might...