I personally don't like reviewers who say "I would image..." or "I would assume" when talking about lower spec model of the same computer, specially when they didn't even test it. If you're the kind of person who has many applications open at once and like to push you computer to the max then sure buying the high end model makes more sense. However, I bought the the lower spec of the 27" iMac (Late 2014) and don't have issues with lag, but then again I don't have many applications open at once.
I'm a graphics designer who also does his own development. A good example is that I use NetBeans for my IDE and I can say it's a memory hog. On my old computer it took a couple of minutes to setup and the computer would bog down if I opened Photoshop and a couple of other applications that I needed. On the iMac the application opens right up and the computer doesn't bog down if I have multiple applications open. Now if I were to go crazy and open up tons of applications that might be a different story. I also don't do a lot of video editing on this computer, but what very little I have done it does a nice job.
Bottom line is if you're the type of person who likes to have many applications open at once or like pushing you computer to max (the highest resolution for example) then yeah it's probably would be better to spec out the computer. However, if you're the type of person who has 2-4 applications open then you should be OK and you can push the computer without any problems with one (two at the max) applications open without any problems then you can get by nicely with the lower spec. To answer your question I personally don't notice any lag. That's my opinion.
