OverRun's post is absolutely right, and he/she does indeed sound like a Professional Airport Engineer.
The point he does not make is that a longer RESA does NOT necessarily mean a safer runway.
Going back to basics, the RESA is there mostly for the accelerate/stop case, from V1. (Yes, there are other reasons, but that's the crucial one).
In that particular case, without the jargon, you work out the weight you can start the T/o roll at, so that when V1 comes up you've still got enough room to stop safely within the remaining paved runway plus the RESA, if you decide not to take-off (using the TODA), and to stop instead.
If you extend the RESA from 90m to 240 by going further out into the country from the existing runway, you simply move the point at which V1 is reached another 150m along the paved surface from start of roll. Now, if you reject the take-off at V1, you hit the RESA surface with 240m of your stopping distance left, instead of 90m, and therefore at a much higher speed and in a heavier aircraft than you would have with the 90m RESA. (Other things being equal etc etc).
As OverRun says, many airports cannot physically extend a RESA, and therefore shorten the T/o Run Available by ending the take-off run 150m before the end of the existing paved runway, thereby reducing operating weight and speed at the point where the paved surface ends. That's OK, and safe.
There's another factor working here. The good book says that RESA's must be able to support the heaviest aircraft using the airport without damage to the aircraft. The whole philosophy of having a RESA depends on that being the case. You, in your fully laden A380, B747 or whatever, should hardly notice as you go off the paved surface and on to the RESA. Your braking action should be unaffected by the surface, and the wheels should not sink in by a centimetre, because doing so would stress and damage the undercarriage..
In your dreams. Few if any RESA's are even built to these standards, let alone tested. If they were, the airport operator would probably simply extend the paved surface with the appropriate bearing strength, because that's not much more expensive.
If you want to check this out at an airport of your choice, try and find, beneath the layers of shopkeepers and accountants, the person who understands RESAs and aircraft safety, and ask him or her what the RESA bearing strength is, when it was last tested, wet and dry, and who by. If they are expecting the A380, this is a very relevant enquiry. At many small regional airports in UK and Europe (to my knowledge, perhaps also elsewhere in the world?) you would get a blank look.
For new airports, one of which I was recently involved with, all the planners are doing is looking at the maximum accelerate/stop distance they can use/need, and then paving that less 240m at each end.
The beancounters love it; no performance penalty and much cheaper.
With ultra-heavy aircraft now in normal use, and an even heavier one on the way, it seems to me that the philosophies that worked for DC7s need a review.