I think it's safe to say that this thread is hitting nerves on a bunch of my pet peeves.
Basing one's evaluation of the validity of the course rating and especially the slope rating on one or two plays of the course represents a HUGE misunderstanding not only of what a handicap index measures, but also the very purpose of the handicap system itself. Not only is your index an expression of your potential, rather than any sort of a predictor of your score on a given day, but the purpose of the system is to allocate strokes for competition, whether formal or informal.
So IF the course or slope ratings aren't accurate (a premise, btw, that I do NOT accept!) it doesn't really matter because ALL players are impacted on a given day are affected by the rating the same way. If I am only allocated an away course handicap of 5 at Bushwood from the white tees instead of the 8 that I think I should be getting, it doesn't matter, since my opponents/fellow competitors are impacted exactly the same way.
An additional mathematical reality about golf is that scratch golfers and very low indexes tend to have their scores in a relatively tight range, which higher indexes shoot scores that vary MUCH more widely, which is the main reason that the slope system was devised in the first place. So when a 15 or higher index (or really even a 10, for that matter) goes to a course and posts a good score relative to their course handicap, and then goes to a second course with a similar slope rating and the same course handicap and shoots a million, what has happened was mathematically completely predictable, and in NO way a commentary on the validity of the slope rating of either course.
When a golfer says, "I'm a 12, and my course handicap was a 10 at Bushwood, but I shot a 93; the slope rating there is MUCH too low!", he has set up a whole bunch of straw men and then knocked them down. His real comment should be this: "I shot a 93 at Bushwood with a course handicap of 10, so I played poorly. However, if I play Bushwood from those same tees 19 more times, I'll probably have 4 (or so) good rounds where I actually play to the 10 course handicap. For the other 15 rounds, I won't play to the 10 because that's just who I am as a golfer. Which is to say, while I DO catch lightning in bottle every now and then, I'm not particularly good on my "average" day, and I really suck on my bad days."
That, of course, is NOT how we like to talk about ourselves, so we construct false narratives about handicaps not traveling well, or course/slope ratings being wrong, and so forth.
Think your handicap doesn't "travel well"? Travel more and get an index that reflects that. Think the slope rating MUST be wrong because you didn't play to your course handicap? Instead of blaming the slope rating and your course handicap, play that course more (like 19 more times!) to get an accurate picture.