Like others have said, look into the MC519 system. It's not cheap, but it's the easiest to use and holds a lot of grass. Here's a picture of our MC519 on back of the 455. The powerflow system does a good job on sucking up the grass and blowing it into the cart. 19 bushels is a decent amount of storage, it is easy to dump, easy to maneuver, and easy to attach/remove. Our MC519 is about 18 years old and we haven't had to replace a single part on it yet.
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We also have an Agri-Fab 26 bushel Mow-n-Vac that we use when doing leaves or the really heavy stuff. The 6 horsepower engine on the Mow-n-Vac has extremely strong suction and also shreds the clippings into extremely tiny pieces. I estimate the 26 bushel Mow-n-vac actually holds twice the material as the 19 bushel MC519 because the clippings are extremely small and dense. Unloading the Mow-n-Vac is a pain in butt, and attaching it isn't that simple either. It is also top heavy when full, so you have to watch out on hilly terrain. After using that for a 20 hour weekend, you will appreciate how much better designed the MC519 system is.
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Personally, I would never get a rear mount bag type catcher. Especially if you own a X595 with 54" deck. I think Deere makes a 7 and 9 bushel capacity grass catcher for the older X500 series, but those would fill up so fast, you would probably quit using it after the first season.