From my testing comparing v4.6 to v4.8, I only saw the transcoding fps drop by around 3 on average. Not bad. In most cases, artifacts were reduced with the newer model. In rare cases, frames contained new artifacts and are slightly worse than what v4.6 produced. As I said though, this is rare. v4.8 is certainly an overall visual improvement. I did not notice any major difference with my thermals, but I did not measure. I just checked during a v4.8 test and the numbers I was getting seemed similar to what I remember getting previously with v4.6.
It is also worth noting that I measured slight savings in file sizes with the newer v4.8 model. My guess is that this is related to how video data is saved and processed. With less artifacts and random, inconsistent splotches of stray pixels, there are less pixel changes to store between video frames. This leads to minor reductions in the size of the outputted file. On a 3-pass interpolation test, I got a file roughly 10% smaller in size. For normal transcoding usages, I wouldn't expect 10% though. Maybe others can comment on their results and correct me on the logic here if I am wrong.
I just wanted to update and say that I performed visual testing with v4.9, and the results were, in most cases, worse compared to v4.8. In frame comparisons, v4.9 won in only a few cases, but v4.8 was far more consistent and preserved more details in both 2D and 3D scenes. It seems like the consensus on here is that v4.9 provided some performance improvements, but it seems to have worsened the visuals in my testing. Maybe coincidentally, my previous theory seems to be holding up, as v4.9 transcoding outputs yielded slight file size increases rather than decreases.
EDIT:
Forgot to specify that I was running v4.9, NOT v4.9 v2.