A lot will depend on your evaporation rate with your boil. I personally boil on the aggressive end, add my sugars in this case the D - 90, during the last 10 minutes of the boil. I will typically get about 10 extra gravity points. As an example a 1.060 Gravity with only Barley Malt will turn into 1.070 with the addition of extra sugars. A lower evaporation rate will likely allow for fewer gravity points.
Used for a Belgian Dubbel, which came out great, regular sugar or brown sugar is just not the same and wont leave the same profile as the syrup does.You can substitute sucrose for the light or table sugar on an IPA or a Belgian single with good results, but if you want to make a Belgian Dubbel or Dark Strong get the good stuff..This is it.
Every year, one of the craft breweries in our city hosts a Meet the Brewers event. This year I had the opportunity to go and meet virtually every professional brewer in the city. One of them brought a Belgian Quad to the event that was excellent. I spent time talking with him about brewing, because he'd spent much of his career training on Belgian brewing techniques.
One of the pieces of advice he shared was that, given the choice of adjuncts to use in a Belgian beer, use Candi Syrup instead of the rocks of Candi Sugar. Before that advice, I'd used both and was disappointed with the beers that used the rocks - but didn't know why. The syrup does seem to impart more flavor, and it's much easier to add to a wort than the rocks.
If you want a little extra flavor kick in a Belgian brew, you can actually use this as your priming sugar when you bottle. Doing that took a mediocre Dubbel I'd made and turned it into a decent one. If I'd used this at the end of the boil, I suspect it would have been great.