OK, in response to my own post above, I redid the simplest test with a Watt meter and took photo's. It takes under five minutes and pretty much proves the 1645 needs more power!
Download ThrottleStop / i7Turbo, RealTemp and FurMark (which only uses a single thread out of the eight threads available).
Start 'm all up, and
don't let ThrotleStop change your system. Only use it for monitoring purposes.
In a little over a minute and a half, my system pulls more than 90W:
This is where it starts throttling, down to 25% or even 12.5%.
Even when fully throttled down, my system still manages to pull
95W from the wall. This is in under three minutes from starting FurMark! If you leave this running, your PSU will shut itself down due to overheating, I've had this happen in under thirty minutes,
using the stock PSU and no modifications by ThrottleStop.
These tests were done with a brand-new PA-3E adapter, a 1645 and no other software running but FurMark, i7Turbo, RealTemp and ThrottleStop. The 3 utilities were used for
monitoring only.
After shutting down FurMark (I don't like to break my PSU) the system still pulls
45W at idle, with the battery full, so no charging needed.
The lowest I've ever seen it go is 32W, with the CPU/GPU idle, screen on its lowest brightness, keyboard backlight off, WiFi off, BlueTooth off. But why buy a machine with all these features if you cannot use them.
The base power usage leaves very little headroom for GPU and CPU, although other tests I've done indicate that using the CPU only (so Prime95, Office work, Photoshop, etc) doesn't tend to lead to throttling most of the time. It is mostly GPU usage that leads to CPU throttling.
So to summarize once again for Dell, this time with visual proof: the 1645 needs more than 90W to fully utilise the onboard hardware, specifically the GPU. For a machine with a powerfull dedicated GPU this makes no sense. Why incorporate a ATI 4670 if it cannot be used.