I am not so sure you will gain as much as you/others expect with the 1250 over the 950.
Agree with windmaker, to avoid slack lines are more about time and technique, than area
A bigger wing is still easier, correct, more room for errors usually, but I think the gain is marginal over a certain size.
Because, smaller wings are a lot more stable because they at speed dont have the same ponyride, when you put your feet wrong, they just ride along a lot more in control.
So the footswitch is just as easy, actually easier than with bigger wings.
So with practice when you have your kite timing and turn correct, you can avoid slack lines, a tad easier with bigger wings though.
But as the footswitch is easier on the smaller (or more flat profiled) ones, there is a tradeoff.
With my barely 80 kg, I have found that an 600 cm2 wing is small in every way, and more difficult if you ride bigger wings also.
But my 800 cm2 and 1200 cm2 area wings, are the same regarding "jibing ease" for me.
Meaning, I think it is not linear, so from the small wings to bigger wings there are huge advantages, but I honestly dont think this advantage is very big or present if you go REALLY big.
Difficult to say, but the advantage might not be as big as you expect, as you got a REALLY big wing now, 950 cm2.
If you are relatively new into hydrofoiling, well, doesnt matter how many years actually, but if you are relatively new into practicing and learning jibing and find it IMPOSSIBLE - then you are just like the rest of us, and time on the water will eventually make it happen.
We have all been there, and bigger wings helps indeed, but dont forget we all have found it impossible at one time, and later possible with even the smallest wings
If you KNOW you like to go slow, and ride waves only, then no doubt go for the 1250.
Otherwise I would hesitate or not do it - but let time make everything work one day, after hard work untill then.
It is NOT easy, as simple as that
I often find the medium size a lot more fun than the big one, eventhough I am not into speed in any way