Anton Yelchin is a good actor (you can check him out as Chekov in the Star Trek movie - which I was luckly enough to watch last week) and we've had many actors replacing others for characters in the Termiantor series, so I think whether he resembles Biehn or not shouldn't effect his performance or the film itself.
ok in t1 reese says that the resistance won cuz their defense grid was smashed. You would figure that an intelligent skynet would operate more that one defense grid right??