Not sure about Lincolns, but I had a mm211 for about 6 months, wanted a longer gun than the 10 foot that came with it - miller told me the drive wasn't built to push a 15 foot gun, that the drive would likely burn out early and that the warranty would be void if I did it. I was working on a brush bucket for my backhoe at the time, it's 8' x 6' x 2' deep, space was limited around it, and a 10 foot gun wasn't working. I'd bought the spoolmate 100 with the unit, which has a 12 foot cable - got by with that, but when I found a mm252 with dual running gear, 30A spool gun, dual running gear and 2 large bottles for a good price, I went for it.
The 252 has a heftier drive and comes with a 15 foot gun. That's the longest standard gun I've seen, you can get about any length SPOOL gun you want since it's not trying to push wire thru the whole cable - the 30A spool gun is named that because it's 30 feet long.
I think that if you're happy with everything else on your mig, you might be better off just doing some sort of jib crane hanger for the mig, centered to give you the most reach -
Of course a spool gun is another option, they're not JUST for aluminum - the downside of that is wire COST - price a 1 lb. spool of S6 wire, multiply that by 11, then check the price for an 8", 11 lb. spool - (I'm assuming your mig won't take 12" spools, not familiar with it)
Just some things to think about... Steve