if you really need that in order to get a boost vs certain threats when facing stall maybe it can be an option, but honestly the main reason why sheer force is such a great ability is that you get the boost with LO without recoil and thus making moves hit extremely hard, which is what, as firehusky said, makes feraligatr worthy of being used over mega gyarados, so personally i wouldn't ever use any other item on gatr
agreed a damage boost of 1.3x is nothing to be scoffed at, here are some waterfall calcs against mew max defence all jolly
gyarados lo -252 Atk Life Orb Gyarados Bounce vs. 252 HP / 252+ Def Mew: 126-149 (31.1 - 36.8%) -- guaranteed 4HKO after Leftovers recovery
feraligatr lo-252+ Atk Life Orb Sheer Force Feraligatr Waterfall vs. 252 HP / 252+ Def Mew: 152-179 (37.6 - 44.3%) -- guaranteed 3HKO after Leftovers recovery
m-gyarados -252 Atk Gyarados Waterfall vs. 252 HP / 252+ Def Mew: 93-109 (23 - 26.9%) -- possible 5HKO after Leftovers recovery
just using mew as neutral type fodder, you can see gatr hits harder than even lo gyarados all jolly, as for adamant
m-gayardos - 252+ Atk Gyarados Waterfall vs. 252 HP / 252+ Def Mew: 102-120 (25.2 - 29.7%) -- 0% chance to 4HKO after Leftovers recovery
gyarados lo -252+ Atk Life Orb Gyarados Waterfall vs. 252 HP / 252+ Def Mew: 133-156 (32.9 - 38.6%) -- 2.7% chance to 3HKO after Leftovers recovery
feraligatr - 252+ Atk Life Orb Sheer Force Feraligatr Crunch vs. 252 HP / 252+ Def Mew: 203-239 (50.2 - 59.1%) -- 76.6% chance to 2HKO after Leftovers
gatr outdamages lo gyarados, with a jolly nature, no recoil damage and adamant was thrown in again to show the big diffrence as well.
All stats copied and pasted from showdown damage calculator