The other thing about 840 vs 980: it's true that the performance of a -10 840 is basically identical to that of a 980. However, operationally, the 980 has two advantages, both related to the engine computers. First of all, in the -10 conversion (690 or 840), you have to manually run the fuel enrich during start. The 980 starts just like a -5-- turn the knob, and monitor the start, that's it.
Secondly, and probably more important, on a -10 conversion, the pilot needs to calculate what the EGT limit is for the current conditions at cruise. This requires consulting a table, and setting a little marker on the EGT gauge. On the 980 (and 1000), the single red line computer does this for you, so all you have to do is keep it below the red line on the gauge.
The SRL does add a bit of complexity, and if it fails or is inadvertently turned off, you need to be very careful to avoid over-temping the engine, but that generally doesn't seem to be a common issue. I've yet to have one fail in many years of flying 980s and 1000s.
The difference between a -5 840 and a 980 is significant. The cruise difference is big, and even bigger is the fact that, basically, you are never temperature limited on takeoff in a 980. You always have the full 730hp no matter what. So hot and high takeoffs in the 980 are a non-issue.