I don't think Invicta has been oversubscribed in recent years. They either have spaces or just about fills. MGGS seem to be the current school of choice though these things do change. I really wouldn't stress, Maidstone has enough spaces for the kids who pass.
only to find that Invicta is way oversubscribed and everyone who was offered a space took up the offer leaving no space!!
I really can't see it being way oversubscribed. I looked at the results for Maidstone and Malling in KM and the pass rate for this year was just about the normal for the area. All the top 5 passes were more Malling area primary schools which keeps demand up for MGGS due to logistics and perhaps one or two other things. There weren't any surprises the side of town close to Invicta, just the usual Roseacre (...and even here a number still opt for MGGS) and then a few other primaries somewhere in the middle.
They may fill up on allocation but then spaces do come up as some people don't take the spaces just like you will be doing for the other grammars. I have never heard a girl in this area passing and not getting in, but there is always a first of course. Its mainly West Kent and Dartford area Grammar's that seem to be oversubscribed in Kent.