That's the million dollar question mate. I would say, it depends on the tracker, the seedbox specs and most of all your budget.
In my opinion you can easily build ratio on most trackers even with a shared seedbox as long as it's 1GBs or more. 100Mbps seedboxes can be good for retention, especially having 1TB+ hard disk, but it will be difficult to build ratio with them.
It all comes down to what you want to achieve and how fast you want to do it. But above all how much you are willing to spend