I believe the only problem would be if you need to open the bonnet without another person to help.
Without the slam panel, the bonnet does not spring up by itself when you pull the cabin lever, you need to pull and keep it pulled, with someone else actually opening the bonnet.
I think its ok, but as previous post, you struggle to open the bonnet.
As someone who experienced this, I had to get some string and tie it around the handle and then pull both the string and bonnet catch at the same time.
Interesting. I have no problem opening the bonnet without the slam panel. It doesn't pop out, but I can still lift it as normal. Not sure if the Vantage S is a little different in that area (although I doubt it).
I've read on here that the slam panel also acts as a heat shield to protect the bonnet from the heat generated from the radiator. So I wouldn't risk driving it for any distance without the slam panel in place.