Well instead of Portugal being ignored in European Wars and left to its own devices, they have suddenly become the easiest target for Spain, I doubt Portugal will survive the 18th century.
If anything we gaining Galicia, which I doubt is possible with any POD after King Denis of Portugal, would strengthening our position.
The only good road in the North to transport artillery was in Castelo Rodrigo and the Northern part of Portugal and Galicia are rocky places, which is a reason why when Spain invaded they usually did so in the south.
Also we were dragged into most of the European Wars, with Spain usually on the other side:
Dutch Revolt check (thanks Spain for dragging us into this mess);
30 Years War check (our men, and the Catalans, were used as cannonfolder by the Spanish);
War of Spanish Succession check (we crowned the bloody Austrian Pretender);
Seven Years War check (such a fantastic war);
Napoleonic Wars check (all of the country burned only to deprive the frogs of supplies; also one of the few wars in which we fought side by side with Spain);