Perhaps my experience is unique, but I did almost exactly as you describe, and it was absolutely painless.
I had a Series 3 HD (bedroom) and HD XL (family room). The plan was to get a new Roamio for the family room, move the HD XL to the bedroom, and sell off the HD.
After I got my Roamio up and running, I temporarily had 3 TiVos on my network. My Roamio happily displayed both my old ones on the My Shows list (the new "Now Playing"). I was able to browse both of them without issue, and transfer them all without issue. I transferred approximately 30 hours of HD content and had zero issues. Regarding it being "painfully slow," that was not my experience. Immediately after it confirmed the first program was going to start transferring, it showed up on my list with a couple of minutes already transferred. Just to be safe, I would queue up 3 or 4 at a time instead of going through my entire catalog at once. Again, no issue. As I said, I'd queue up 3 or 4 of them, start watching something else, and by the time I finished watching that program, all the items I queued had completed. So, I'd move on to 3 or 4 more, a few at a time, until I was done.
I will say, a majority of the items I was transferring was from the HD XL to the Roamio, and both are connected to Ethernet, so that might be why I had such a high success rate. The HD is on a TiVo wireless adapter - luckily, I only had about 4 programs to transfer off of that one, but even that one went alright. For that one, I queued them up overnight, and by the next morning, they were done (I didn't worry exactly how much time it took, I just verified they had transferred successfully).
So, as I said, perhaps my experience is unique, but for me, it worked exactly as designed, and my experience was flawless. (FYI, I did all this with no CableCARD connected to either the HD or HD XL).