You mentioned miniDisplayPort 1.1 and your link says 1.2, probably a simple typo but is 1.4 backward compatible with 1.1?
A Late 2008 MacBook features DisplayPort 1.1. It doesn't matter if the adapter is (mini)DisplayPort 1.1 or 1.2.
DisplayPort 1.1 can do 360 MHz pixel clock at 8 bpc (288 MHz at 10 bpc), but macOS' driver for the GeForce 9400M
may impose a lower, arbitrary limit. I don't have a 9400M Mac anymore but I have one with a GeForce 320M and it can do the full 360 MHz.
HDMI 1.4 can only do 340 MHz pixel clock, so DisplayPort 1.1 is good enough. And even if there's an arbitrary limit to 270 MHz pixel clock, that's still good for 2560×1440 or
2560×1600 at 60 Hz... or 3840×2160 at 30 Hz.
So can you use this adapter with a Late 2008 MacBook as well as with a Mac from 2014-2015 with Thunderbolt 2?
A 2014/2015 Thunderbolt 2 Mac features DisplayPort 1.2 and, if it has an Intel Iris Pro 5200, HD/Iris 6000-series, or NVIDIA GeForce GT 600/700-series "Kepler" GPU, a pixel clock limit of 540 MHz.
If has an AMD Radeon R9 M200/M300-series GPU, the pixel clock limit
might be ≈600 MHz instead.
540 MHz is just enough for 3840×2160 at 60 Hz using CVT-RB timings (which is what monitors use), but you need an active miniDisplayPort
1.2 to HDMI
2.0 adapter to attain that.
If your monitor is a TV, and maybe also if it's a monitor, it's possible that it defaults to HDMI timings for 3840×2160 at 60 Hz when using its HDMI input(s). These HDMI timings require a pixel clock of 594 MHz (because the pixel clock must be a multiple of 74.25 MHz for HDMI timings) and exceed the 540 MHz limit. So you wouldn't be able to attain 3840×2160 at 60 Hz even using the correct adapter, unless you created a custom CVT-RB timing for mode using e.g.
SwitchResX and the monitor/TV accepted it via HDMI (you won't know until you test it
).
So I bought an active miniDP to HDMI adapter hoping that it would solve my connectivity issues. Alas, it did not. Even with the active adapter I still could not get a resolution higher that 1080p.
It's possible that the adapter only negotiated reduced bit rate (RBR) rather than high bit rate (HBR) for some reason, limiting the maximum pixel clock attainable to 216 MHz... not sufficient for 2560×1440 at 60 Hz.
I've had this happen with an active miniDisplayPort 1.2 to HDMI 2.0 (!) adapter and a slightly weird monitor.
