I think that pulling the speedline tight with loaders, GRCS, or pulleys is over-rated. Sure, a tight line is great for eliminating excessive droop, but it also loads down the TIP quite a bit more.
I just have my guys pull the line tight by hand, then hold the slack with our port-a-wrap. Easy and fast, and they can slack the line easily when the branch comes down. The POW keeps the load from ever getting too heavy for one man to control, too.
Using the skid steer to tension the speedline doesn't sound to productive to me: that operator is not doing much if he is sitting in a machine. The machine isn't doing too much either if it is just holding a rope.
If a branch is so heavy that too much drop to clear an obstruction is anticipated, I will add a normal lowering line to hold it, as though the speed line was not even there. Then as you lower the load with the stronger setup, the speedline directs it away from whatever you were over. Nice and slow and controlled. If things are so close that you can't have any sag in the line...perhaps you should cut smaller pieces?
I like these carabiners for speedlines: pretty strong and they shouldn't wear out too quick:
http://www.sherrilltree.com/Professional-Gear/Accessory-Non-locking-/Carabiner-285
Lighter branches I just send down with no roller on a much cheaper carabiner:
http://www.baileysonline.com/itemdetail.asp?item=21358&catID=9643
The cheap carabiners don't slide down the rope as easy, but that's usually not that important. I suppose someday I will really need to pull the speedline real tight, but I have gotten by so many years without any speedlines at all, what I am doing now seems plenty good enough.