Edit: Bah. Think I found the main reason; The built in synthetic metronome of Reaper. I've seen many people mentioning that they use vsti's for this purpose. I now see why. I even did a quick run with a vsti earlier tonight, but I guess I was sloppy then.
Here is the result after just doing a run with Rewired Reason Redrums as metronome instead:
Quite a difference, aye? The weird thing is that the metronome plays along fine with the Redrum pattern. I think its just not as defined or something. Now, I wonder if samples work well instead. I kind of like having that metronome button you know
I'm just happy to know I can keep on using Reaper with a big midi smile on my face.
Edit 2: In all fariness, I maybe just had a really good day when this was posted. This was extremely good results for any quantization. Almost a tad too good to be reproduced in fact, but I have mainly focused on the internal metronome so I wouldnt be able to tell for sure yet. I'm convinced that the end result of using external devices (like vsti's or rewired apps) are somewhat more precise though. Maybe because of an added latency that makes the midi note timing more correct.