I'll reply to the rest of the post shortly, but wanted to answer this question before I leave for work:
EDIT: Still getting quite fast load on dashboard with this dataset. Infact most of the time in the ms range. At slowest nearly a second. What is the point where people are starting to experience serious performance degradation, all the time?
If you've fetched the latest code and applied the database migration script, then you shouldn't be seeing any degradation anymore. If you didn't, then you would certainly see horrible dashboard performance after a while, as there were some missing indexes that would cause table scans against submitted_work during the dashboard query. (As submitted_work tends to grow slower than work_data -- but proportionally to it -- you'll need a fast miner, or a bunch of slow ones, to start seeing the performance issues quicker.)