I think that ideally the same 24s would deliver a 2560x1600 resolution, however I'm still quite happy with the setup as it is. 3x 27"s might be nice, but I fear it could become too much. Well, at least for my wallet, that is!
An issue with the 4k displays is that if you're going to have 3x of them, you're going to hit GPU walls very, very quickly.
aka Assuming monitors last ~3 years and cost below 2,000$ it takes a tiny difference to be worth it because your not just comparing productivity delta's but also cost delta's. In other words a 2,000$ monitor that improves productivity by 1/20th of 1% than a 1,000$ monitor is clearly worth it over 3 years even if the difference is simply eye strain. 3x (salary + benefits + overhead) * .0005 > 1000$.
PS: Another way to look at it is spending an extra 1-2k per year on salary over industry standard rarely tempts programmers but an extra 1-2k per year on hardware might.
That said, I agree. The benefit doesn't have to be big to justify it.