If the 8 Years war end with the Netherlands owning most of today Belgium could this translate in a Netherlands capable of projecting power into the Rhine and North Sea?
It wouldn't. The focus of the Dutch, especialy Holland, Zeeland, Flanders, Brabant and Friesland, the parts that would become the most important was always away from Germany and towards the sea (and maybe France). If the Netherlands would gain Belgium during the revolt, it would gain Antwerp and Flanders, both areas that became rich by overseas trading.
I wouldn´t call Netherlands one of the least German parts of the HRE, Savoy, Italy, Wallonia and others are.
The Dutch principalities always were a border area with an independencestreak, that did not care too much about the emperor with a lot of English and French influences. I will admit that Italy was less German than the Netherlands, but I think you can compare the Netherlands to Switserland. Let me put it this way. If you show a map of 20th century Europe to someone from 1500, they would not be surprised that the Netherlands wasn't part of Germany. They would be surprised that Austria wasn't part of it.
The OTL unification is surely butterflied but an unification is not tied to the chain of event IOTL. It would end with different territories of course.
German unification is certainly not impossible, but it is not a given. A Germany divided by several smaller states (like Bavaria, Prussia, Saxony and Brunswick) is another possibility. Or a Germany which is divided by surrounding nations (like France, Denmark, Sweden, Poland etc).