I think we can agree the obvious intent in the example is to have the spell be treated as a modifier to be applied concurrently as the power fist's modifier. As the OP, I didn't challenge the intent, just how GW got there by their example. Changing characteristics on principle is not the same as applying a situational modifier, and such language should be used judiciously.
The rules don't differentiate these. Any modification is treated the same and are cumulatively applied. Seems easy to go, "this model is getting a +1 from this and a x2 from this and here is the answer". The order of the buffs and where they originate from doesn't matter. It's always the same way to combine them.
Not sure why following the normal order of operations for math is unintuitive.
So the "while" part in the quoted GW by me isn't an indicator that the +1 goes first before the x2 of the power fist... I mean "while" means that it's already in efect when the power fist enters in action... I'm not a native english speaker, so my understanding could be judged by translations issues :/