Didn't bother replying because I was going by subjective measures - how fast the fuel gauge dropped (noticeably faster than usual) and how the car felt under acceleration. Non-quantifiable I know, but I've owned and driven the car long enough to notice when things change significantly. No reason to make things up, as I was keen to make this stuff work.
I ended up blending 1L of 0w20 (Racing 0 - HTHS 2.5cP) with 2.5L of 10w40 (Racing 10, HTHS 4.49cP), which (assuming linearity) should give a cP of roughly 3.92 to match the oil I was running previously. I had enough 10w40 leftover to give it a fresh change, figured a flush wouldn't hurt anyway as the previous OCI was long and dirty.
Fuel economy seems back to normal now, and the car does not seem reluctant to rev anymore. Pretty much feels identical to how it ran on the RP on all counts.
If all is still well after 5k, I will run the same blend again.
I stand by my prediction that the HTHS value has a noticeable effect on performance and economy after this experience (much more than cSt@40/@100 values) - the value for their 10w40 is substantially higher than most 40wt oils, about the same jump as from 30wt to 40wt in other oils.
Edit - I know HTHS probably isn't a linear relationship but it looks like my estimation was close enough, happy with the results.