.. a few additional thoughts ..
(a) the design certification figures (ie 2.4 gross etc ..) are WAT (maximum Weight for Altitude and Temperature) limits (for the various segments) and are intended only to make sure that the certification animal is able to climb at a (very) modest gradient (think just how shallow 2.4 really is) after an engine failure according to the Hp and OAT .. if you prefer, the WAT limited weights avoid the need for climb to be dependent on the curvature of the earth. WAT limits have nothing to do with obstacles or anything else .. they are pure and simply minimum free air climb gradient requirements ..
(b) the SID requirements are procedural .. and, often, very much obstacle driven ....
(c) heed OS and OzEx re making sure that you can get the OEI gradient above the SID to avoid the bumpy bits ..
(d) for a twin under critical conditions .. it is not unheard of for the net flight path to extend upwards of 50 miles ...
(e) all in all .. if the airport is not excessively benign .. then one is well advised to leave the number crunching to the ops engineers and fly the OEI escape procedure as prescribed ..