In principle, the idea of compensating the developer's exhaustion with time extension would lead to more problems. Using Blix is also a prerequisite for problems. But Tetenal has done a lot of research and testing and found some reasonable compromise. However, these kits are intended for the home hobbyist, not a professional with full control. If you scan the negatives, you probably won't see any difference.
Still, using it at 38 degrees reduces the potential problems by a factor, and I'd stick with that scheme.
Tetenal marketed their blix based C-41. and the 3-bath E-6 originally as press kits in the early 1980´s, when they entered in the market. Most of the modern home kits are copies of these - some are better, but some have been known to use shortcuts. Blix can lead into problems with film, and I personally don`t use any of those kits, and use Fuji chemistry instead for my color negatives and RA-4 papers. Combined bleach fix is used in commercial settings only with paper. The E6 3 bath process has even more issues, but this is an another story.
Ideally, the developer should be either used only once or alternatively replenished for optimal results. That said, moderate reuse with a small adjustment of times will produce negatives that are within the limits of acceptable results published by film manufacturers. I don`t have the equipment for analyzing test strips, so I can`t provide with exact numerical data on this matter. This does not mean that using one liter of developer for 20 rolls was ok, although some hobbyists with low expectations claim they can use their cheap-o kits for 30 rolls or more. I would probably agree with Fuji Hunt claiming you can use their 5L kit for approximately 12 films per liter without replenishing, but for "perfect" results, one-shot usage of the developer is recommended.
For CineStill or similar chemistry,, it is hard to say anything definite, as many of them seem to be nonstandard, with developers working in different temperatures with different times. It means no data by Fuji or Kodak concerning the standard process will apply. Tetenal used a standard developer for their C-41, with only the bleach fix being their own invention. This is not the case with many of the modern home kirs. This is also a reason to avoid using them.