IMHO, it may make it seem sweeter... but the amount of sugar will be whatever amount you choose to use!
PS: once again, like I have just posted... I have accidentally boiled tea... And, it helped... It makes it less bitter, as it pulls the full flavor and body from the tea leaves. One can not over-steep tea.
In fact, if the water is not hot enough, or the the tea is not steeped long enough, one will be left without the full flavor, only the bitterness that initially comes out. You don't HAVE to boil it. (as one web-site I saw actually defined southern sweet tea as 'boiled'.) but if you are not steeping in simmering boiling hot water. You aren't getting the full effect.
I do believe that it must be sweetened when warm/hot.
Cold tea is just not going to dissolve enough sugar, and really pull all of the sugar and tea together to get that rich flavor.
Whenever we are up north, or even in Florida, where they only serve unsweetened tea, and not true, presweetened southern sweet tea... it is just awful... You can put sugar packets in that cold tea and stir and stir forever.... You'll never have good, sweet, southern, tea that way!