Burton is already 88kg (I suspect that is not correct and he is already heavier) and at 6'3" if he has another off-season he is likely to get to 100kg. That isn't a problem if he can retain his agility and speed over 40m.
Since 1980, the tallest 100m Olympic champion was Usain Bolt at 6'5", he weighed 94kg which is also the heaviest weight. What is interesting is that there has been several 6' plus champions including Christie, Lewis, Gatlin and Bailey. The heaviest of these is Christie at 92kg and the lightest was Lewis at 80kg. Clearly, if he is given the right training Burton could be fast. He will need to do the sort of explosive weight training Christie did if he needs to get stronger for the NRL. If he puts on too much weight, even if it is muscle, he will turn into Mansour, slow down and limit his effectiveness as a half. History doesn't lie. Fast means relatively light if you are tall, stocky if you are short.
Burton's weight will be a massive factor In respect of agility as well. A bigger mass is harder to move and his ability to twist, step and move laterally will become more difficult the bigger he gets.
I say all this to point out you cannot predict what your genetics will deliver. He might grow another two inches. He might have the physique of a Cheetah, a la Carl Lewis and Usain Bolt or the powerful broad build of a Linford Christie, Ben Johnson type. Whatever it is he will need to be strong enough to take the impacts so he isn't injured all the time. At the moment he sort of looks like Steve Menzies in build and not the type who gets really bulky.
Once this genetic lottery plays out and provided he gets the right training, he might still be nimble and fast enough to be a half. If not, then a fullback, lock or second rower is more likely. Brad Fittler moved between lock and five eighth and he was 6'1 and 96kg. He was never fast but his ball playing made him a pretty handy player. If Burton keeps developing his skill but the genetic lottery slows him down, he could be a brilliant lock and still get paid a mozza. I agree that second row wouldn't utilise his ball playing and kicking skills. Why would anybody do that? Greg Inglis was a 6'5" 155kg fullback, so Burton could play that position as well. Greg wasn't fast, but he was powerful and skilful.
I'll go out on a limb here and say the club won't try and turn him into a second rower and will want to utilise his skills at lock, five eighth or fullback.