If you looked closely at the last couple of 2K/4K groups, you may have noticed a small improvement that's been making its way into the upscaled components. Slidebars are one of those UI widgets, like scrollbars, that are resistant to being rescaled due to complexities in how they take input and display their state. If you think about it, scrollbars and slidebars have a lot in common.
There are not nearly as many slidebars in the UI as there are scrollbars, and fortunately the "thumb" control can be sized up independent of the widget without creating any new graphics... sort of. What you're seeing is a little bit of sleight-of-hand. The thumb controls are actually an enlarged copy of the thumb floating over top of a normal-sized control. The track doesn't get sized-up, and that's as much an aesthetic decision as a technical one. Normal-size tracks do their job at all sizes without scaling them up, which would cause the entire control to lose its crispness.
There's a line or two of code involved in each slidebar instance, so each one has to be evaluated separately to keep "real" and "fake" thumb controls in agreement. As a result, I can't do a single sweep and convert all of them at one time. Over the coming days if you check for updates and see new file versions in strange places, it's likely the L and XL sizes are getting thumb control renovations. I think Darq Siders who use Large or X-Large versions will like these controls better. They look more consistent on L and XL surfaces and they also give you a more appropriately sized screen area to grab with the mouse.