I think the brochure says 16 kg on the back will still handle "Sweet"
I haven't carried that much on mine, neither do I intend to, around 12kg certainly becomes noticeable to me, it's OK most of the time but out of the saddle becomes borderline unpleasant.
I'm using Ortlieb front rollers and a Carradice saddlebag. Both of which pre-date the Mercury as does the Tubus rack with lower rails. Heel clearance is tight, but even with my size 11's there's enough with the panniers on the upper rail and obviously more on the lower. I think the larger rollers, for me, would be too much both in capacity and size.
My tent is an inner first pitch and packs into two stuffsacks, these are normally inside a pannier but can be attached to the tops (As photo below, not on Merc) this frees up about 2/3 of a pannier if I need it for something else.
(https://farm2.staticflickr.com/1729/41920213474_0f4d95f88f.jpg) (https://flic.kr/p/26Sm3Zh)
Panniers (https://flic.kr/p/26Sm3Zh) by Paul (https://www.flickr.com/photos/phbike/), on Flickr
Couple of photos of my Mercury loaded in this thread
http://thorncyclesforum.co.uk/index.php?topic=12921.msg96745#msg96745