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.