Drupal has been reported to run slowly on DreamHost shared servers. The combination of heavy MySQL usage, the fact that the MySQL servers are on different machines than the web server (extra network trip), and the inability to control what other users on the machine are doing all have to do with that.
Some of that is probably attributable to DreamHost’s structure, but a lot of it also has to do with the fact that it is “shared” hosting.
With the MySQL demand you describe, I think you are absolutely correct about that. Such is the nature of shared servers. If you want complete freedom from those concerns, obviously you should be looking at a different type of solution, such as a dedicated box, or even a less powerful Linux-Vserver (like DreamHost P.S.).
Sure, it would be nice to be able to have snappy performance with heavy MySQL usage, with the bandwidth and space DreamHost offers, at the current DreamHost price. but the reality is that the kind of usage you describe is at least “ambitious” for such a set-up. Why set yourself up for disappointment? IF the low price is what is attracting you, you can always give it a try with the 97-Day Money Back Guarantee, but that’s a lot of work if you already anticipate it is not going to work for you.
Like I said before, I would not expect to get acceptable performance at the level of usage you describe, using Drupal, on the DreamHost shared server plan. Of course, YMMV and my opinion probably doesn’t mean much as I have only hosted here for several years and am not likely to know what you could reasonably expect.
By the way, this is primarily a customer-to-customer forum and, while they occasionally drop in, DreamHost staff rarely post here … but you never know. Maybe you will get an “official dreamhost opinion”.
You could also ask them “officially” with a pre-sales information request, via their Contact Form. Good Luck!