Changing autogrow settings with database mirroring
I'm addressing the issue using Policy Based Management. I created a policy on the Data File facet that states all files must have @GrowthType = KB. 

I also filed a connect item: 
https://connect.microsoft.com/SQLServer/feedback/details/796354/autogrow-is-percent-flag-set-on-principal-does-not-persist-to-mirror

UPDATE: This is fixed in SQL Server 2008 R2 SP2 CU8 and SP1 CU13.