I've got the standard MCS for the 2520/62D and the hydro version would be nice. The hydraulic MCS isn't (wasn't?) approved for the 2520 because the 2520 was too light, but a nice dealer may be willing to order it for you (plus I think it can be easily modified to be 3PH mounted rather than drawbar - makes things easier to install). I have the standard PowerFlow and it's not bad for most of my uses, clogging isn't a big problem even when vacuuming up leaves last weekend, but it has issues with long grass and sometimes I'll hit a twig just right to get it to jam under the deck (not sure an auxiliary motor would help with that).
What you pay for to get the hydro dump set are easier 3PH install rather than lifting the drawbar mounted bagger in pieces, on-the-seat clipping removal, larger capacity, and a supplementary motor to reduce clogs. Points for the standard 3-bagger are price, official support, no secondary motor means less maintenance, and I haven't found a need to have any front ballast for mowing my fairly flat property (at least beyond the FEL mounting frame with BH brackets).
I had seen the Cyclone-like solutions and they seem nice, but I'm completely incompetent with maneuvering a vehicle with a trailer so I hadn't seriously considered one. Those who have them seem to be typically happy with them, though.
I thought you had mentioned some rarely mowed areas (I could be mistaken - it's been a long week!). I'd probably look at a rotary cutter or flail mower for that. A MMM will struggle on its own and you likely won't be able to successfully use any sort of bagger if it's like the vegetation by me. A rotary cutter is nice in that it's cheap and can be sued on underbrush as well. A flail mower produces a better quality cut and typically lays down the cuttings much neater, but it's pricier.
Yet another option I wish I had considered more is to get a standalone lawn tractor or ZTR - let the 2520 be a tractor instead of an overgrown mower. The downside of this is that I wouldn't be using the 2520 nearly as much, but on the flip side because the mower gear is on I tend to let the projects build up until it's worthwhile to get off my lazy posterior and get the tractor implements on.
Hope this helps!