Gasoline has fewer BTU's per unit measure than diesel, so diesel will most always produce more power per gallon
ICE's at best, the last time checked, only utilize <50% of the energy potential a gallon of fuel consumed, so there is head room to improve both power and MPG
Science will continue to discover new laws of physics that will improve power and MPG
*ALL* crude graphs, reports, etc (to date) listing the end of supply has fine print..."known reserves"...and they are finding more reserves all the time...but it will end...just not as soon as they say...
Power has the bottom line of ultimate and actual PSI on the piston tops. Factored by the area of that piston top. Limiting factors are mainly with both material science and thermal dynamics. Toss in manufacturing science