I am looking to get a roll cage for my rc monster truck. I have a few questions about them.

How do they perform under high speeds?
How much of a difference do they have while off a jump?
How do they perform on a plastic chassis when taking a hard landing?

IMHO roll cages look funny, and don't add to the durability of the vehicle, and will take away from the performance. Since most are made of aluminum, it will bend when taking hard roof landings.

And they look like a pain to get on And off.

How do they perform under high speeds?
Perform? There is no performance in a roll cage. How would a NASCAR perform with added weight?
How much of a difference do they have while off a jump?
That would be "functionality" it could save parts, it could break parts. Weight would/could make it jump differently.
How do they perform on a plastic chassis when taking a hard landing?
Again, you don't get "performance" out of a roll cage. Your lid, the plastic body the truck came with is your "roll cage"..

I have a roll cage on my emaxx, and for that particular truck, the roll cage I believe made a big difference in the reliability of the truck in the fact that it tied the front and rear of the truck together to help against flex between the bulkheads and the main chassis. I didn't notice any difference in the performance of the truck, it's a monster truck, not a precise racing machine. The cage I have is made of stainless steel bar, so it's not really easily bent, it'll bend by hand, but it takes a fair amount of persuasion.
My experience with the roll cage I had on my emaxx may be quite different than other's experience, which it sounds like, was quite positive, and I feel it saved the truck a couple times.