What modifiers applies for casting a Powerbolt/ball on a vehicle/structure?

The RAW says that you need to beat the objects structure rating at 4+ for vehicles.

Is that number fixed or does the "+" implies that it can be higher depending on vehicles mass etc?

It's supposed to increase based on complexity. The fluff excuse being that the more processed something is the less residual Mana there is left in it and the harder it is to affect with magic.

I can see an argument for SOTA Military gear maybe going up to OR 5 and High Tech Alien stuff being OR 6.

By the fluff there has to be a limit though, I would say 6, where an object contains no residual Mana at all and it can't get any harder to affect after that.