I would lean more toward heli-coiling. Regardless you are going to want to make sure you are at least 1cm hole center to hole center. The benefit to using heli-coils for this is if you are a mm or two off from where a hole already exists you can "cheat" and offset the heli-coil drill bit long as the existing hole falls inside the diameter of the larger heli-coil bit. However it all depends on how swiss cheesed and close together the existing holes are.
In general I've found T-Bolts are a pain in the ass. I have a buddy who is 6'7" and snowboards so i have had to add T-bolts so he could widen his stance and it's always taken more effort you would expect. If you are T-bolting mark your pattern carefully, drill straight through the ski with a small drill bit. Cut the P-tex away around the holes slightly bigger than your T-bolts and drill the hole to the correct diameter for you T-bolts. Use 80 grit sand paper to rough up the exposed layer of fiberglass under the P-tex. Using generous amounts of epoxy, I use marine epoxy, insert the T-bolts. Let them cure for at least 24 hours. Mount the binding and make sure the T-bolts are solid. Fill in the P-tex and get your bases stone ground.
