It is a big misconception to think that QC are some magical device that can speedup arbitrary computation. Evidence suggests that they have limited speedup applicability
(all variations of Grover's unstructured search and Shor's group structured search).
I think I urged you to read Scott Aaronson's
"The Limits of Quantum Computers"
http://www.scottaaronson.com/writings/limitsqc-draft.pdflast time, to correct this misconception.
Yes, and I quoted text that was proving my point.