TheiC - you make many valid points and I hope to elaborate more on a few. Being early can be just as catastrophic as being late, it all depends on the segment you are now in compared to where the system predicted you to be. Some of my areas are affected by danger areas that when active high level, vastly reduces the flow rate as it takes 5 separated routes and forces all that traffic down just one single airway. You fly slow just before activation and then you turn up in the first period of activation and add to the restricted flow. Similarly, you get a slot in the first period after the activity finishes and turn up early, you then are in a more severe flowed segment and cannot fly the anticipated route as the danger areas are still active, the ATC workload can be huge.
Sadly there currently aren't any perfect answers. We cannot get the camera vans out to say "stop speeding/Oi fly quicker". I only use the Spanish/French directs as that is my experience and I have seen first hand the consequence. There are two airlines I regularly work who fly speeds vastly different to other operators of the same aircraft type, however the system processes the same speed for them all. The difference in flight time is significant. Sadly my 6 minute saving is about 4 aircraft an hour out of the 60-80 aircraft the sectors work. I'd love to be able to climb you to your optimum knowing where you need to descend again to conform with flow restrictions, however very few European systems inform the controllers where/when you need to be descended or which sectors you are flow restricted through. Having system compatibility means having systems that can talk to each other, which is currently very limited and unlikely to improve in the foreseeable future.
At night that simply depends on the controller, if you asked me, I'd make the calls and check the system, but it's not always simple or clear.