Any suggestion on extending CFGid to include calculation capabilities? Or should I just avoid CFGrid and look for other JavaScript-based grid libraries?

Avoid cfgrid. I've been happy with DataTables for most of my grid requirements.

Toss cfgrid in the trash; go with datatables

Avoid anything CFML implements for the UI. It all lost any relevance it had by the time Adobe largely gave up on it all after... CF8?

Datatables.net is very much the way to go.

In this case, datatables is quite good. I use it a lot. grid.js is also nice (and lightweight, and integrates into several client-side frameworks as well).