The only validity I would argue is to gain confidence in the final readpoint and that you weren't just lucky.
You don't say what T/C conditions are to determine how much 250 cycles are wrt to your pass/fail criteria. For example, if you T/C conditions are -65C/150C and duration is 500 cycles (A Q100 Grade 1 semiconductor), then additiona 250Cycles are x1.5 your qual duration.
If it is lesser conditions - say -40/125C 1000 cycles, then it is x1.25 qual qual.
One thing with T/C is that they may be passing but are already fatigued and about to fail. You may only be testing at room temp but if you tri-temp tested, then you may see failures at either hot or cold temp. Doing Extened T/C with just a room temp is one way to guardband against that.
Did your customer request this upfront or are they springing this on you now? If latter and you had prior agreement, then you may have grounds for saying no. Also what is your track record of issues with temp cycling? If you have been seeing problems before, you may want to do additional cycles to assess it's robustness. The other counter argument of doing 250 cycles is time. If you have dwell times on top of the temp cycles, then this can add time.