I can honestly say that I've never typed the Bitcoin symbol in my entire life. If someone outside here sent it to me it might take me a few seconds to figure out what they're referring to.
Traitor! You infidel!!

But seriously, I don't see people using it.
If it's the same as on Android, where you have to go to symbols, tap and hold on the $ and then select
BTC it's a pain in the ass
I would type faster "bitcoin" than going through all of that.
It will really turn to a positive impact simply because some IOS user will notice the symbol of bitcoin and afterwards they tend to search it and they will try to buy bitcoin. So if they tend to buy bitcoin the demand on market will increase so the result is high price on market.
As I was saying above, you first need to find it.
But if you honestly think that somebody who has an iPhone and who has probably heard about
BTC a thousand times before and decided not to buy, is going to do so now because of a fancy symbol on his keyboard you really need to get in touch with reality.
What do you get from adding a bitcoin currency symbol to iOS?
Nothing.