I'm still early on, but I was afk for a while and came back to a scrap amount well over my cap.
I think the tooltip explains... storage caps are just softcaps... you gain 25% of normal gains past softcap... still better to increase storage caps when you need to... to continue gaining at 100% speed... and since you can't buy things that cost more than your storage cap, even if you have the resources for it...