I can see your reasoning behind the use of the fat gripz Pistachio, and I think that if you increase your reps, like you said to 5x5 than you will prob increase your muscle mass. As long as you go as heavy as you can for 5x5 with fat grips, than you will be fine, so 140 may still be a bit light for you if you can pull 200. I can pull 160x1, and use 130 for 5x5 on my deads. But given the fact you will be using fat gripz, 140 may be a good starting point. Give it a go mate, don't injure yourself, remember more gains come from consistent lifting, injury only stuffs consistency, if you feel you may injure yourself continuing to train a certain way than stop. No-body knows you better than YOU.
Graeme