Is it unethical to hire a paper writer AFTER the assignment is done?
In your hypothetical situation, no cheating occurs, since you do not misrepresent your own efforts. Comparing your own to someone else's solution after corrections have been returned in order to better your understanding is a perfectly good study practice.

Let me also clarify one point: you say, "paper writers [...] known for getting (dishonest, of course) "A"s on all the papers they write". There is nothing dishonest about the A grade. If the teacher gives it an A grade, it's because it is an excellent paper. If it had been handed in with a different name on it, it would have received the same honest A grade. The only dishonest thing is the student's behavior in putting their own name on it and claiming it as their own work, and arguably the ghostwriter's in knowingly assisting in this deception. Absent this, everything is above-board.

The only point where questions could be raised is the influence of money in your hypothetical. There are two places where this could play a role:

1) Acquiring this solution is expensive, and therefore out of reach for poorer students. This kind of disparity seems to be more acceptable to some societies than others, as shown e.g. by the price of education in the US (lots) vs Germany (nothing).

2) You are contributing to the maintenance of an industry which is predominantly assisting others' dishonesty. On this question, which is similar to e.g. contributing to bittorrent development because you like to use it to download linux distributions, I think that society has not formed a uniform opinion, and it is left to the individual to decide their own comfort level.