In my opinion: flaked wheat is not a passable substitute for unmalted wheat.
Maybe its an issue with freshness in flaked wheat, or something that the process of gelatinizing raw wheat at home contributes, but I have always found beers made with raw wheat to be far superior.
A cereal mash is not terribly complicated. I think its easier than a single decoction.
Torrefied ('puffed') wheat is another option. Never tried it in a witbier, but I like it as a substitute for flaked wheat in smaller quantities (small amount for head retention, body, etc.). It has a subtle, fresh popcorn flavor.