I meant to reply to this when I first read it on Sunday...
In principle, renting a long lens is a good idea, especially since you are in a tire kicking mode with right of return privileges.
I would caution you, though, that the 5" plus front element diameter "exotics" are, under the best of circumstances, and on top of the best of support, more difficult/temperamental. They will never act like a 70-200 or 300/4. I'm thinking the 500/4, 400/2.8 and 600/4, or equivalent.
These larger lenses will always have a long recovery time after a tap test, and will always display some movement in the viewfinder, even with good Long Lens Technique.
A Series 5 is significantly better than a Series 3, but without one to test side by side with the Series 3 legs I'm not sure how you can fully evaluate it.
I am not suggesting you bring in yet another tripod for "review", but just explaining that your perceptions may be misleading. And in particular, it takes more than a day or two to learn to handle these lenses. More like months to really get good at at and comfortable, and get reasonably consistent results. Probably longer than the time you would want to rent a test lens.
Now, if you use VR, then that big lens should be far easier to manage and the results "out of the box" should be better. My comments above are more related to testing without VR.
The entire subject of VR with these long lenses is a complex one, perhaps with as many opinions as users . Plus I do not have VR on my older long lenses (that is why I could afford them ). So I don't like to to talk about VR performance much beyond a very conceptual level.