На сколько % снижается производительность системы при использовании ECC памяти?

Using ECC decreases your computer's performance by about 2 percent. Current technology DRAM is very stable, and memory errors are rare, so unless you have a need for ECC, you are better served with non-parity (non-ECC) memory. 

Источник: http://www.crucial.com/kb/answer.aspx?qid=3692