I've tried this out and it is a disappointment. 8.1 doesn't actually switch the rendering of the app to a different percentage scale when switching displays, it just scales the app rendered at the previous scale.
What happens is a "target" scaling percentage is set - any monitors that roughly match that percentage/DPI get a 1-1 pixel mapping of how apps currently render at 125%/150% etc. Monitors that have a greatly different DPI (for example, a Surface Pro internal screen) then have a scaled app. For instance, the app renders at 125% (the "target" percentage) and is then scaled down or up by the graphics card to the scale percentage for displays that don't match the target percentage. This is never a nice pixel double or halving that OSX carries out, but always a blurry mess, scaling up or down. The taskbar is not scaled at the moment either, rendering at the target percentage scale on all displays, so you get a mini or a large taskbar on the mismatched display.
They would be much better off rendering everything at 200% and scaling down, like OSX.
Could you explain what you mean? In Windows The GUI itself is scaled but it is still rendered at the native resolution of the display. Resampling virtual resolution pixels to physical pixels is the OS X approach.
Edit: It appears that Windows applications that don't support the High DPI interface are rendered then resampled as you say. So it's up to the app developer to support scaling, although you can disable the app resampling behaviour on a per app basis too.
What happens is a "target" scaling percentage is set - any monitors that roughly match that percentage/DPI get a 1-1 pixel mapping of how apps currently render at 125%/150% etc. Monitors that have a greatly different DPI (for example, a Surface Pro internal screen) then have a scaled app. For instance, the app renders at 125% (the "target" percentage) and is then scaled down or up by the graphics card to the scale percentage for displays that don't match the target percentage. This is never a nice pixel double or halving that OSX carries out, but always a blurry mess, scaling up or down. The taskbar is not scaled at the moment either, rendering at the target percentage scale on all displays, so you get a mini or a large taskbar on the mismatched display.
They would be much better off rendering everything at 200% and scaling down, like OSX.