I agree with everyone. Please just buy a set of re-050 for rear. I was in your EXACT position. Part of me wanted to put RE-11 in the back as my rear wear out much faster than front. but resisted the temptation. Put a set of re-050 in rear. By the time rear was bald, front were bald too. Then changed all 4 tires to RE-11. I was in heaven with RE-11S!
2nd best investment I made for the z (HKS exhaust being first). No mix match. But a pair of re050. my