I don't really think this card is that op, even if 6 are used. Yeah it can summon a ceature with +10/+10/ every 4 turns, but two phase recluses (also costing 8 aether) would do 14*4 or 56 damage before the creature came out and unless the mutant did more than 4 damage before the buff, the two spiders would still out damage it. A phase dragon would put out 40 damage before the mutant is summoned and remains a 10 damage source that cannot be targeted, however the mutant most likely is targetable by things such as rewind and freeze negating its power.
Like scaredgirl said, the 4 turn wait greatly negates the scaryness of this card. It would take atleast the second turn to summon this card with a good tower draw and then 4 more turns before a creature is summoned. The second portal would take two more turns to summon. A good rush deck should win in 5 turns every game, so under ideal circumstances the mutant should never see the field. Yes there are dim shields, but several cards counter those already and if they play dim shields it will take a number of turns to get out the portals. the good thing for aether is that decks probably won't have enough deflags for this and the dim shields both, however if the dim shields are killed this would not be as big of a worry. It would stink to have this card stolen first turn out though.
All in all, I don't really see the portal any stronger than anything Aether already has, but would be a good card for those that like luck and random based decks. In this respect it almost seems like it should be an entropy or time card, but Aether still works. It would make a nice addition to the game; I'd like to see more big, balanced cards added to the game.