If you lap both CPU and Heatsink, the benefit becomes pretty apparent as you increase TDP (assuming imperfections)
Cooler was a Xigamatek Loki, pretty underwhelming, especially for a BD chip.
This was a long time ago, and I only did it because I was curious and was going to sell it anyway. Most improvement was likely from CPU lapping.
8150 was lapped down to mostly copper finish with varied grit from 400 to 2500 on glass. CPU IHS was noticeably concave, you can tell easily from where the copper begins to show during the lapping process.
Because this was so long ago I don't remember exact details and I didn't do any hardcore data collection, but IIRC temperature dropped approximately 5c down to 48~50c under load across cores. Ambient temp would have varied 23~26c, but should be mostly consistent. Slight undervolt and overclock to 4Ghz.
Even 2-3c difference is a big deal for an AMD chip.