It'll give you some sort of blue results.. maybe blue with copper leakage (similar but not the same). But blue and blue copper are separate color patterns and as such splash and splash copper would play out the same.. still with the copper or not.. See if this thread gives you a better answer...
Straw colored feathers.. You're right.. he's not perfect for breeding to the standard.. But sometimes you start with what you've got.. and for crossing over the other hens is completely irrelevant.
Please note that crossing a Splash Marans is not the same as splash copper and will not likely...