It's impossible to predict engine life but it surely won't go from 100% to 10% (unless something breaks). Running 40% means the engine can burn more fuel since nitro contains oxygen. More fuel burnt also means the engine will actually get more lubrication.
Rule of thumb :
More power generated --> less life
More rpm --> less life
An un-optimized engine running 40% could make the same amount of power as an optimized one running 30%. To take advantage of 40% and make the most power it takes a bit more than adding shims. Just saying it will not necessarily make more power because there is more nitro in the fuel.
