I've had pretty good luck with DSL providers in three locations in the bay area and one in washington state. Sync speeds have always been at the top of the range, and usually upload/download speed to real destinations is consistent with the sync speed. Depends on the destination of course. And a lot of people on DSL have poor outside wiring (or it's just a long distance) and don't get great sync speeds or the lose sync when it rains and the lines get wet, etc. Mostly though, outside of exceptional circumstances the DSL data rates are slow enough that the bottleneck is either the individual subscriber's line (outdoor and indoor), their equipment, or wifi problems.

When I was on ATT gigabit fiber in San Jose, it was hard to confirm the speeds, but they would vary a lot more. I think most of the time at peak I would only see 400 Mbps down, but closer to the line speed off-peak. Upstream was more consistent.