If your basis on doing 13 units is solely based on wanting to acheive a 99+ ATAR and think that by having more units you have a greater chance of getting it, I wouldn't recommend it.
However, myself doing 13 units, I did not find it difficult it all. Obviously, those people with less units will have more free periods and more, but at the same time doing 13 units can be beneficial.
The reason why I didn't drop anything is because I needed the pressure and the stress of always having another subject to do.
I hope this makes sense, but always knowing that I had another subject to study for, I tended not to procrastinate. This is based on what type of person you are. If you are someone who needs a push, or can't seem to motivate yourself, I would recommend. It's not a crazy option, just one that would require dedication.
+ you might as well TRY how you can cope with 13 units, and if it doesn't work, no worries! Just drop!
You'd much rather stay safe, than be sorry.