Depends. Looking from a "soft" point of view, he might find having all that kit motivational or pleasing in some way.
My productivity was certainly lower when I was a PC user, as compared to the Mac. I really enjoy using my Macs and don't find work to be so grating when the system keeps out of my way and the hardware is solid.
Could I produce just as well with one machine and a smaller monitor? Sure, but I probably wouldn't be as jazzed or enthusiastic about getting started.
The study did find that extremely large monitors are less effective (this might be an ergonomic artifact. They didn't differentiate between one doubleplusbig monitor and two singleplusbig), so you'd be on firmer groud if you'd said:
"Could I produce more with ... a bigger monitor than what I've got now? Maybe yes, maybe no, but I'd definitely be more jazzed or enthusiastic about getting started."
You make a good point, but I don't think it's true for me. I just spent the last two weeks only using my MBP in the lounge, leaving my Mac Pro and 30" ACD gathering dust upstairs :) I'm now back upstairs, however.. it's all about the variety in my case.
I bought a 30 inch this summer and I love it. I really do like coming home and working. Now I need to setup a tiling window manager because it is an annoyance dragging windows all over the place.
i guess not, it could be diminishing return
or even negative return (more distractions, harder to admin / sync / whatever)