well, i'm an all-grain brewer, but i can't imagine at all that there could be any difference in flavor when adding one of the malt extract compounds later than the other. extract is extract, no matter if it is a powder or a syrup.
but there is a difference concerning the hop additions. you can even add a part of the hops before the boil (at 78 °C / 172 °F). in this case the hops leaves more very fine flavors in the beer. But as hefeweizen has only approx. 10 to 15 IBU, you can't really taste the hops anyway. the flavor is only determined by the malt (extract - light or dark), the yeast and the CO2 content.
Regarding clearing agents like irish moss i recommend to not use them, for you want a real hefeweizen to be (heavily) cloudy. here in germany - not everybody but many people - move their bottle of hefeweizen to get the yeast off of the ground. (By the way, the cloudiness doen't only result from the yeast but also from the protein.(
hopefully, my coment helps you a little bit.
good luck with the hefeweizen.
let me have a rating of your beer when it's ready.
best regards, grain-pa