Looking to up grade speakers. KEF Reference 201/2 vs. WILSON BENESCH ARC.

I've never owned the KEF 201/2, so I can't help with those, but I do own the WB Arcs currently and am listening to them as I type this.   I use the Arcs in an 11x10 room and they work beautifully.  In my larger room, around 16x17, I prefer my other speakers, a pair of BMC PureVox.   If it's any help, or maybe you've owned some of these, I can tell you that in my space (and I compared in both my 11x10 and 16x17 rooms), I prefer the Arcs to Proac D2, B&W 805 and B&W PM1.  Without resorting to hyperbole, I can tell you the difference between the Arcs and the Proac D2 was not minor.  So maybe if you're familiar with the Proac monitor sound, this comparison will be helpful.

What size room do you have?  And have you listened to the Arcs yet?

That room is perfect sized for the Arcs.  The Arcs sound different because of the steel and carbon fiber cabinet - they disappear so easily, but for some people they disappear too easily.  You lose that slight coloration that comes from wooden cabinets. 

To a lot of listeners, that coloration warms the sound, which they like.  I think this is why you'll often hear people describe speakers like YG, Wilson Benesch, Magico, and others that use metal cabinets as "clinical" or "analytical" because the speakers are a little too transparent for them.  It may sound counter-intuitive, but not everybody wants to hear exactly what is on the recording, especially if it's a mediocre or bad recording.  

My reaction was overwhelmingly positive - I liked the sound so much, and the effect of removing the wooden box from the sound, that I went and bought more speakers with non-wooden cabinets (the PureVox).  I want my speakers to completely disappear in the room, I don't want to hear any coloration.  It's true - bad recordings sound bad on the Arcs, they don't mask anything and they don't "help" bad recordings, but good recordings sound wonderful.

The difference between the Arcs and the PM1 - which you do have access to for reference - is huge.  I've done this comparison in my home, and at a dealer, with the same result.  PM1 sounded like it had a wet blanket thrown over it.   You just have to decide what sound you want - the transparency of the Arcs is amazing if transparency is what you're after.