16cm.....6.5". VS commodores are pretty much the same as VN VP commodores WRT parcel shelves. Check out
this guide, with pictures. Pictures have been taken out of first post, but scroll down there are some there in a later post.
They should simply slot in. Even though the holes are 6", I got Pioneer 6.5"s in the back of mine, simply slotted in, lined up appropriate holes, and screwed back down. Easy as.
You can put the parcel shelf back if you wish. Probably a good idea, as it is just bare metal underneath that. If you are feeling adventurous, you can cut out the existing grills and put the ones that you got with your speakers in with some modification of the parcel shelf.
Major advantage? Better sound. You also can have some control over how it sounds - you can position the tweeter in a different spot to the woofer, so your highs can sound brighter/better. Also the bass (actually, not entirely bass but sub-bass) produced by the splits over say some 4" is much more.