The eXt73 patch-set is designed to offer greater performance and reduced power usage on an otherwise generic Linux kernel image. This is my first time hearing of this work, likely since all development work seems to be done in Polish as is their project site. A Phoronix reader this morning pointed out this independent article (Gooogle Translated to English) from dobreprogramy.pl that is testing out the patched kernel compared to mainline.
The benchmarks they run -- largely using our Phoronix Test Suite benchmarking software -- do indicate that the eXt73 kernel is generally faster than the mainline kernel. The cost of the patches seem to be 50 Polish Zloty (or $14~15 USD) per system.
The project site for this work is netext73.pl. There is NeteXt73 on GitHub that does have some open-source code. It appears the cost of the patches may be just for those wanting a patched kernel in binary form. Hopefully the Polish Phoronix readers will be able to chime in on the forums with a more proper translation of this work and other details on this interesting eXt73 kernel initiative.