
There's absolutely no reason why differences in the DRAM access time 
_alone_ would cause an incompatibility.  There would have to be another 
difference between the SIMMs for there to be a problem.  I've often used 
memory of different speeds with no problems whatsoever.  As long as it's
as fast (or faster) than the minimum requirement you should be fine.

Just out of curiosity, did you actually try this and see a problem, or 
were you told it wouldn't work and so never tried it?  Also out of 
curiosity, do you know exactly who in Cupertino you dealer talked to (as 
I'd like to find out what they're basing this recommendation on).