I did have about 48 INT earlier, but disbanded most for better range FTR. Those and the 12 STR took China from 137ic to 49ic in just 5 weeks. It reached zero IC perhaps a week or two later. This resulted in massive "low on supply" modifier in all ground battles. Unfortunately, several hundred rockets will never achieve that because - unlike bombers that can be directed at only industrial damage - the rockets are "strategic generalists" and IC damage is just a small part of the overall damage they spend their force on. While you will certainly get a major effect given how many you have, unless you get the IC low enough, the AI repairs it quickly.
I sense that you underestimate the effect. You got a reduction of 64.23% in 5 weeks with Str1943. It has strategic attack 20, the V2A9 has 80. That is still only half of a ICBM, but quite something. More importantly the rockets cannot be stopped or just reduced in their effectiveness. So their is a chance that they do more damage in say 5 hours than you did in 5 weeks. But at this point that is just speculation.
And all those partisans in Portugal territory over two provinces away from my frontline could still affect things for me?
It is not impossible. Partisans reduce the flow of ESE through their province. If however you are talking about provinces that donnot matter for your ESE at a given set of provinces deemed important, than you can ignore it.
The obvious fact is that partisans do decrease ESE even if there is no tc-overload. Where they decrease ESE to which degree is a bit complicated. My formulation "Partisans reduce the flow of ESE through their province" should capture the essense of it.