For an SXM-CDG flight, and A340 will by flying with far less than a full load of fuel for an 8 hour flight by an aircraft that has an maximum endurance of around 14 hours.
While the A340 does have a reputation for having a slow rate of climb this relates more to time taken to reach initial cruise altitude, rather than initial climb performance on departure. The A340 may often take quite a long time to reach cruising altitude, but on departure it must meet the same obstacle clearance limits as other aircraft.
In fact, in determining takeoff performance the A340 will have a distinct advantage over twin-engine types.
For a 2-engine aircraft, maximum weight out of a particular runway is invariably limited by takeoff distance required in the event of an engine failiure at V1, from which point the aircarft must accelerate to VR and climb away on only half the available engines.
On the other hand 4-engine aircraft is much less likely to be limited by this, but by the normal all-engines takeoff distance required multiplied by a safety factor of 1.15 (1.15 is the JAR figure - the FAA may use a different one, just covering myself
In summary, a 4-engined aircraft is likely to be allowed to takeoff at a weight which reflects it's performance on the day, a 2-engined aircraft will have it's max allowable takeoffweight reduced from what it could thoretically handle on all engines to allow for the much more severe effect of an engine failiure on the twin.
As for the mountainous terrain, its again likely that the 4-engined aircraft has the advantage in obstacle clearance, whatever the normal situation, an A340 on 3 engines will outclimb a 777 on 1 engine, and this again affects the conditions under which the aircraft can operate.
Let's not also forget that St Maarten is also at sea-level which will also help with performance.
I hope this little explanation has given some insight into why the A340 operations out of St Maarten aren't really that big a deal...