I'd be really interested to know what changes in particular pushed Streatham High Road over the edge. I was reading through this blog post
BusAndTrainUser Verify and south bound at least it seems to be an issue with cars pulling out onto the high road at Sunnyhill and Pinfold Road, and potentially Prentis Road, and then further down there seems to be more traffic at the lights at Stanthorpe Road, although that should clear more easily as there's a box junction and light phasings.
If the only entry onto the high road at the particularly bottlenecked section (from the Odeon down to Aldi) was via Pendennis Road and Stanthorpe Road only (where there are traffic lights), and other roads were made one way and left turn from the high road only at their entry points, I wonder if that would have made things a little bit better. I think it would mean the gate at the end of Valley Road would need to go, but you'd most likely get fewer rat runners on the road overall, as you couldn't pass through the entire length of the road, and it would be such a faff going down Valley and coming out at Pendennis that I imagine drivers would just carry on Leigham Court Road or go via the High Road.