Just that most grads who do honours get in compared to people who apply without honours, not sure how true this is though
I don't actually know what the ratio of honours and non-honours are in medicine but don't do honours for the sake of GPA. This is particularly the case for those who already have competitive grades. I am fairly sure there is no significant causal relationship between the two anyway. In my opinion, it's a dangerous thought to think that doing honours would increase your chance. This would only work if you somehow managed to get very good grades in honours - I don't know who said honours is easy but this sure is far away from the truth. It's a different ball game. It's harder. It's less predictable. It's different from what you have been doing.
As Ruth said, it really is more important to think about your employment prospects. It's a hard life outside of university. You don't want to end up being like some applicants who put zero input back-up plans during their undergraduate study and have nothing to fall back on.
That being said, if you are genuinely interested, there is nothing stopping you from trying it out. Honours is generally a good programme to take if you have the opportunity.