1920 x 1080 Resolution @ 27" good enough for Sonar UI? Waste of money to go higher?

If the 27" 1920x1080 is usuable (comfortable)  for you,then you have a baseline to compare.
1920 / 27 = 71 dots per in. (horiz).
If you actually measured your mon's horiz width, this would be more accurate, but this is valid for a ballpark comparison.
To go to 2560 and have the screen objects remain the same size (but have more visible real estate),
2560 / 71 = 36.
So you would need a 36" monitor to keep objects the same size as now but on a larger desktop.
 
If you got a 27" 2560x1440 then
2560 / 27=94.
Objects would be 71/94 or 0.75 times as large on a 27" 2560x1440 (compared to 27" x1920) but on a larger desktop.
 
As far as touch goes, there are more variables having to do with the resolution of the touch screen.
If the two monitors being compared have a similar touch technology and resolution, you can do similar calculations to get a ballpark comparison of the usable touch resolution.

microapp
Also be aware that Sonar likes the Windows text size to remain at 100%.
I am not aware that this has changed anytime recently.
So you cannot adjust the size of screen objects in Windows.
It makes sense to try to estimate the object sizes in relation to your current setup because you can only change the object sizes by adjusting the screen resolution. 
If you get a hi-res small monitor and the Sonar controls appear too small, then the only option is to make the res less, so why pay for the hi-res if you have to lower the res to make it usable.
 
 

I can verify that this not the case anymore.

I had a 30" 2560 x 1600 monitor.  For me, this was the perfect size for Sonar, as you got good horizontal and vertical resolution, but Windows did not need to scale over 100%, so things stayed very readable and very sharp.  I recently "upgraded" to a 32" UHD 3840 x 2160 monitor.  Problem is, at this resolution, you need to scale Windows to make anything readable.  Sonar's UI does not scale particularly nicely, so you end up with things looking a little fuzzy.
 
So, yeah, I think the sweat spot is 2560 x 1600.  For me, vertical real estate is important, so not sure I would gain anything from a  2560 x 1440 monitor, but I guess it's a personal preference thing too.

rezab

microapp
Also be aware that Sonar likes the Windows text size to remain at 100%.
I am not aware that this has changed anytime recently.
So you cannot adjust the size of screen objects in Windows.
It makes sense to try to estimate the object sizes in relation to your current setup because you can only change the object sizes by adjusting the screen resolution. 
If you get a hi-res small monitor and the Sonar controls appear too small, then the only option is to make the res less, so why pay for the hi-res if you have to lower the res to make it usable.
 
 

I can verify that this not the case anymore.

 Yes, confirmed .

I'm using 2 x 27" monitors at 1920 x 1080 and Sonar is fine.
 
I could go higher but the text becomes a little small and as mentioned above I don't want to cock up Sonar's delicate sensibilities by changing font sizes.
 

Perhaps, rather than having a higher resolution monitor, it would be better to have more monitors. I use 3 monitors, where normally on the central monitor I leave the track view, on the lower resolution monitor I leave the console view and on the third monitor I work with Piano Roll View, among others.

A great thread on the move to larger 4K screens.  I will be updating my three 27/28 inch hd screens to a Phillips 40" 4k in the new year... everyone who's done this ain't going back... my tired eyes need this clarity and screen real-estate is vastly superior... 
 
May still use an hd or two for side info but for the main screen, a 40 4k seems the best bet at the moment... and can be had for under $800ish these days... Have seen some nice 50 inchers also...

And for anyone considering multiple screens - be aware of issues with different resolutions / DPI....
I've got a Surface Pro 3 for work (which has a stupidly high resolution - can't remember exactly what it is). I use an external 24" screen that is good (DELL) but nowhere near the same DPI. You have to find a compromise between screen scaling settings in W10. You can set different scales for each screen, but you get very strange artefacts when moving windows between screens: when a window spans both screens, the screen that has the larger proportion on it determines to scaling.  This means that as you drag from one to another there is a point when the scaling suddenly changes (which is very off-putting).
I don't know they could overcome this - I think the answer is to get multiple screens with the same DPI - but this is not easy to do.
(There is also an issue where you can set different screen scalings for docked / undocked - but they won't take effect until you log off/on - so I have to log off every time I undock which is very frustrating).

paulf707
You can set different scales for each screen, but you get very strange artefacts when moving windows between screens: when a window spans both screens, the screen that has the larger proportion on it determines to scaling.  This means that as you drag from one to another there is a point when the scaling suddenly changes (which is very off-putting).
I don't know they could overcome this - I think the answer is to get multiple screens with the same DPI - but this is not easy to do.

I use two screens of differing sizes (24" and 20"), but I leave everything set at 100% scaling.  When I drag from one to another everything stays the same, no artifacts or anything else odd.  I assume they must be similar DPI.  Just more dots on the larger screen; more inches = more dots.

I use a 4K 55" TV and its great to have a console and track view more than a meter wide in front of me :)