I'm well-versed in manually-partitioning for RAID0 or RAID1, but only with BIOS, and I would just give /sda an extra 1MB partition for GRUB.

With UEFI installed, however, I am not sure what to do with the ESP partition: just set aside 500MB on /sda or should I RAID0 250MB each on /sda and /sdb?

EDIT: I am using a 2012 Mac mini, which does not have a hardware RAID card. Your results may vary.