My name Eric Klein and I am the developer of K-Cycles. I am professional developer in graphics for many years and took mathieu course over 2 years to learn about coding in the Blender environment and have recommended it. This course used the Lukas Stockner patch on scrambling distance from the blender foundation and optimize utilization for Cuda.
The key to performance in both in E-Cycles and K-Cycles is using scrambling distance patch with high GPU optimizations. That is it. We can say that E-Cycles RTX is using Lukas Stockner patch on scrambling distance from Blender Foundation for the performance improvements. Easy to test just set noise level or scrambling distance to 1.0 and see how performance is reduce.
K-Cycles is not the code from the course. The old course only cover Cuda no Optix or RTX. I don’t think is usable now since the Blender master has change so much. K-Cycles does use scrambling distance, but that is based on Lukas Stockner patch which the course and E-Cycles RTX also use. The optimization for high GPU utilization in Optix are different K-Cycles RTX needs large tiles and E-Cycles RTX uses regular size.
I have been doing my custom build for couple of years already and describe in detail in my post from a couple of months ago see 2x faster with scrambling distance and high gpu utilization.
The basic scrambling distance and cuda/optix optimization are available now for free at Graphicall. Search for the Bone-Studio build. He has repository and thanks to Juan Gea Rodiguez for offering this build to the community.
I had hope that Blender Foundation would do some of this optimization, but it does not look like is going to happen in near future. I decided to make available my custom RTX build to the community at an affordable price. After doing polishing and adding some new features including a fast viewport and beta testing for a month I release K-Cycles. I does take a bit work to maintain multiple builds, to constantly adjust the code because the blender master is changing, release updates and customer support. I will be adding more unique features over time.
I have no issue with mathieu if he wants to release E-Cycles RTX an affordable price it is currently $300, since my benchmark comparisons were done using Optix and E-Cycles RTX. These benchmarks are fair using identical scrambling distance value, render samples and default settings.