Blazing fast speeds on a shared seedbox server?

YES, that is possible! and not only with SSD servers, but also with more traditional servers.
You need something like Pulsed Media's Super100 seedbox, with few users per disk and traffic limits ensuring you get a fair piece of the pie that is bandwidth.

You will then be able to use much more larger share of the pie at the time you need it, rather than potentially be limited by resource hungry other users on the server, and thus getting blazing fast seedbox speeds.

Look for the overall offer, if it has traffic limits you can potentially have much higher burst performance than otherwise expected.