I gather you're extrapolating from the use of "dead" as an intensifier in other expressions such as "dead certain" or "dead on" (as well as "dead right", "dead on time", etc.) where "dead" means "absolutely" or "perfectly". In English as in any language, you can apply the idiomatic use to a new context, but you may confuse your audience.
I would understand what you meant by "dead impressed", especially if spoken with positive intonation, but I would assume it was slang from somewhere other than where I'm from. A similar example is the Boston (US) use of "wicked", as in:
My boy's wicked smart
Given the previous dialogue from the linked movie, it's obvious even to someone not from Boston that "wicked" means "extraordinarily". In the same way your use of "dead" is fine, but it depends on the context.
Note: You may not find examples of "dead impressed" as an idiom, but instead try "dead sexy" and there will be many.