The answer to "is it worth it" is "it depends". Sometimes Disney has package discounts with dining that you can only get if include the tickets. This includes the infamous "
free dining" packages. I have no idea whether you are thinking of trying to get a package, but you would in that case not want to pre-buy.
As far as how much it will go up, well we already know that now right? What's interesting is that that it shows:
Four day $350 -> $400
Five day $370 -> $415
So if you look at the previous prices, after four days - each step was previously $20 a day (5 day $370, 6 day $390, 7 day $410, 8 day $430)
But the step from 4 to 5 days dropped to only $15. So you would expect 5 day $415, 6 day $430, 7 day $445, 8 day $460.
So this would say the 3 and 4 day passes are taking the worst hit.