How long is "gradually"? I will love to know.
The main purpose of the test is to know whether the redirected domain is going to gain it's power back to rank another domain.
I don't care to argue the merits of 301's on overall ability to improve SEO / SERP, but think about it logically.
Backlinks are like a river network. You have numerous streams flowing in to rivers, which then flow in to larger rivers, which then flow to the ocean (some money site).
Now imagine that someone cuts off the flow of those streams (the 301). Eventually, the larger river will dry up, right?
Now imagine that whomever cut off the flow of those streams had actually just diverted them to another river network. That river network is going to see higher water levels, right?
To somehow claim that a 301 would continue to boost a site after being removed seems absolutely ludicrous. That's point one.
Point two, will the 301 continue to be able to rank other sites?
Using the river analogy, if someone upstream started throwing trash & dumping toxic chemicals in the water (think GSA / etc.), people are eventually going to stop drinking the water.
On the other hand, if the water is coming from pristine mountain streams then what's the problem?
My point being, I think your experiment is interesting, but there are too many variables to factor & that experimenting with a single 301 won't prove anything. It may work, it may not work. Try it with 15 or 20, with various ages & backlink profiles & then I think you'd have a more valid test.
