the key factor is timing:before spanish can train troops in navarre and valencia,they must attack,siege,and rebulid first. you can attack spanish before they get benfits form their new land.

if spanish can train X troops and you let them take navarre and valencia. then we assume that spanish send 800 troops to attack these provinces. after the siege there, they left 350 in valencia and 200 in navarre.then you attack castile at this time, spanish have to left at least 100 troops in both provinces and the force they have is X-450.

another choice: you take the navarre and valencia
firstly it is hard to take both provinces before spanish. secondly if you can take both, you will loss at least 250 troops( as the spanish), and you have to keep more troops than spanish to prevent rebell (in hard/expert)
if you can train Y troops, then the troop left to attack castile is only Y-550.

X-450 vs Y and X vs Y-550, the gap is 1000.