I've had my TFV8 for some time now, don't hit it every day or for long periods, but do like it for when I want a bit more. I was getting along just fine with the X4 coils up around 115-120 watts. Put a new coil in the other day, same as I had many times. It started getting really hot at 100 watts and was unvapable above that. I cranked it back down to about 85 watts, still on the warm side for me, but will get a little mileage out of it. I am not sure why this coil got so hot, but seems the air flow is rather restricted for some reason. I don't mind playing around with this coil a bit, but will change it out after a bit and see if a new coil works better. It may just be a bad coil or part of a bad run/box. If I can make them work at any wattage, I'll use 'em up and move on to the box of T8 coils I've got in stock.
Sorry, really not much help for you other then as others have said start at a lower wattage and move up in steps until it starts getting too hot or starts losing flavor. Enjoy it there no matter where it settles in for wattage, And now, I figure even if I put in a new coil, it may not hit the same wattage as the one it replaced,