Performance on updating row
If you are trying to decide whether to build a dynamic or CASE expression, omitting columns whose values haven't changed, I'd say that it depends on your index structure, and you will need to test. In this article, I found that with a single, simple, clustered primary key, the blind update was always faster; this changed in some scenarios when multiple indexes were added.

http://www.sqlperformance.com/2012/10/t-sql-queries/conditional-updates

Where the breaking point is, I'm not sure; nor did I test with dynamic SQL.