How well you can oc a 8600K is determined by your luck in getting a good chip.
as of 3/22/2018
What % of I5-8600k chips can oc
at a agressive vcore of 1.4 or so and delidded
4.9 96%
5.0 85%
5.1 66%
5.2 38%
5.3 13%
You are doing fine. No need to change anything if you are happy with 4.5.
The impact of delidding is to reduce the temperatures of the chip. Perhaps by 20c.
If you increase the multiplier, your vcore needs to increase also to remain stable.
It is the increased vcore that drives the temperature up.
Monitor your vcore with cpu-Z. It is excessive vcore that can damage your processor.
1.4 is about tops, I would not worry about 1.3.
If you use adaptive voltage and speedstep, your multiplier and the associated vcore will reduce when there is little to do.
Many testers use AVX instructions which are not common in gaming.
Such a stress test will show about two multipliers lower than what you could get in actual usage.
I would not get too serious about perfect stability if you can tolerate a shutdown failure.
Should that happen, just assume your oc was too aggressive, adjust your oc and move on.
On a stress test, 85c is probably a good target; that is what the default is for OCCT.
The cpu will throttle by itself or shut down if it detects a dangerous temperature.
That is around 100c.
I think if your normal load temperatures are in the 75c. range, that is ok.