In combination with OIS the IBIS will lead to more keepers but it is not a certain deal, it will just make it more frequent. Many times you can get similar results if you take few more shots in rapid succession. Yes, you will have to sort through more shots later but you will have it there, the sharp one. This is valid for somewhere in the 1/5-1/30 range, slower than 1/5 the IBIS will have the upper hand as in you will get some shots sharp. I kept hearing about multi seconds sharp shots but I rarely can make that happen.
Which lenses do you use that you feel the OIS is not helpful? Keep in mind that with the X-T2 I need a faster time than with the X-T1. Overall I feel they add 2-3 stops to what you would get without. I have the 10-24 (most effective, can get even 4 stops occasionally), 18-55 and 55-200.