Late to the party, but here's another data point:
Scored 6.5 on the memory test. I'm quite certain I could do better if I had rested--I was still winded from lifting a bunch of heavy stuff.
Just started SSiW about a week ago, and after 5 lessons of Course 1 (each repeated once or twice), I got the email advice saying to not repeat lessons and to do Level 1, etc. I've finished 17 Challenges so far with no repeats, and no pausing. (Hoping to finish Level 1 in the next 2 days or so.)
I find SSiW quite "difficult," done according to Aran's more up-to-date instructions, in that it requires more concerted effort than anything else I've tried, but I think it's working well. I'd say maybe 3/10 difficulty. I definitely feel like my brain is melting, and I'm hanging on for dear life. But putting in the effort, it does what it says on the tin. Every now and then I'll peek at the written form for a word I'm having extra trouble learning, and that helps it finally start to stick. I "see" words in my head when I speak, even when I don't know how to write them, so figure I can at least try to see the correct word.
I've been a compulsive language learner for decades (but never studied Welsh before), so I can compare to lots and lots of other techniques. I've always been one of those "visual learners," which is part of why SSiW feels more difficult. But it also feels hard because it's very effective at creating "controlled chaos"--we're intentionally made to lose control and get overwhelmed, but not SO overwhelmed that the lesson no longer works as a teaching tool.
Hope that helps!