Efficiency question: VARCHAR with empty string vs NULL

Hello,

I have a table with about 15-20 VARCHAR items. I use VARCHAR on the 
assumption that it is more space efficient and not too much slower than 
CHAR, especially when you expect many of those VARCHAR to be null or empty 
strings.

Is there a performance difference with VARCHAR elements of value NULL and 
zero-length string? If so, can I assume that NULL is the better performer, 
and thus re-code my INSERTS to replace empty strings with NULLs (or vice 
versa)?

Now that I've had to re-write all my data to substring it to fit in the 
VARCHARs, one more "thing" won't matter much.

Many Thanks,

Doug