The phrase "considerable upside" means that there is a significant potential for improvement, growth, or positive outcomes in a particular situation or investment. Full definition
He is a developmental project at best with considerable upside if a team is patient with his progress. (ndtscouting.com)
There is still considerable upside potential in the ADRs (Jae Jun reflects on it here), and long - suffering holders of this former net - net may yet see their investment thesis vindicated. (longtermvalue.wordpress.com)
Some also see considerable upside for the retailer's flagging stock. (canadianbusiness.com)