Good chance next years rMBP will come with usb c and tb3, and we might even see a 5k thunderbolt display alongside. Why not just get the rMBP and hook it a to the 5k monitor with greater versatility considering desktop cpu improvements and imac gpu improvements have slowed to a crawl? Skylake iris pro could be close to the M380, a new dgpu on rmbp plus the possibility of egpu support from tb3