Your statements that:
Well, I would have said that with a downhill slope you will accelerate to V1 faster, thus reaching V1 in a smaller distance which should allow you to take-off with a higher weight than with a zero slope runway.
Moreover in case of engine failure before V1, you will have more runway to stop the aircraft (... I understand that because of the downslope you will need a greater distance to stop the airplane but you do have that greater distance because V1 was reached sooner).
Are OK but they miss the point of the question.
The key piece of information in this question is that
the aeroplane is tyre speed limited.
This means that to avoid bursting the tyre we must ensure the Vlof is not graeter than the tyre speed limit.
So the question now becomes "What affect will a donwslope or upslope have on Vlof?"
The answer is that runway slope will not affect Vlof so it will not affect the tyre speed limited take-off mass.
Now a question for you:
Would it not be better if you were to to use a QB which includes lots of detailed explanations?