T-Bolting

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.

awesome help. thanks