I am currently using a 29" 1080p IPS Ultrawide (21:9) monitor (Asus PB298Q) stacked on top of a 32" 1440p 16:9 VA monitor (Asus PB328Q). Here's a photo of the monitors (during installation):
It depends on your personal use of such monitors. For me, I use the 32" main monitor for gaming as well as computer-aided design/graphic design works while the top 29" ultrawide monitor I use for PC monitoring (CAM, Afterburner), Web Browsing, Excel Spreadsheets, and/or Full-Screen Movies.
I have a fairly large/deep desktop where both monitors are approximately 2 feet (main monitor) to 2.5 feet (upper monitor) away from my eyesight. Based on my current use, I find the upper 1080p monitor too small at that distance when viewing texts (but okay for watching movies).
The screen size of the 29" 1080p ultrawide (27.5" wide x 11.5" high) is about 3/4 of the screen real estate of the 32" 1440p 16:9 monitor (28" wide x 16" high). It has more-or-less the same pixel density, but not exact, so icons/texts on the ultrawide appear almost the same size when dragged into the bottom monitor.
In your case, as you already have a 27" 1440p (approx. screen measuring 23.5" wide x 13.25" high), your planned 25" ultrawide 1080p (screen at 23" wide x 9.75" high), will be exactly the same proportions as the monitors I have (just a bit smaller). Pixel density of your 27" 1440p and 25" 1080p will be around the same (0.23mm) so texts and icons will also appear the same size when you drag objects between the monitors.
As for visual acquity, it will depend on how far away you will be viewing your screens. I you have a smaller desk than mine and your monitors are positioned about 1 foot or so, then, you'd have better viewing than having such monitors farther away.