A bit of both. I know that Obstacle clearance was responsible for the 6 degree glideslope on rw 28/10, but RTOW restrictions were both field and obstacle based. Cant remember exact factors for both since our 146 RTOW performance books have long vanished. EI had its own performance figures which were generally far more conservitive and limiting than that found in the AFM, so in theory we probably could have taken a few more pax.
Best ask a current City Jet lad who would be far more in the know than i am.
I'm currently on the 'Bus' and have forgotten a lot of the 146 performance stuff.
oh and yes, i do remember playing with the figures to try to get as many of the pax out as possible. Sometimes we were juggling down to kilos!!!