I wouldn't ditch the crystal malt. I use 4-6% crystal malt in my big beers,both barleywines and RIS. I like the flavor it brings to the beer and tend to mash around 150 to increase the fermentability. I've never had a problem reaching 75% ADF. Some of my beers have done okay http://wiki.homebrewersassociation.org/2010Barleywine
Good point here. If you're mashing the Crystal malt (especially at a lower mash temp), then you aren't adding a boatload of unfermentables as is common belief. The enzymes in the mash convert the dextrins in the Crystal malt just the same. Darker crystal malts will leave behind more unfermentables, but not as much as you'd think.
To the OP - I do agree that cutting back on the number of different Crystal malts is probably a wise choice, just to reduce the potential for muddiness. I like your choice of the Extra Dark English Crystal. I've got a RIS coming up and I was leaning in a similar direction with my recipe as you. I'm thinking of splashing in a small amount of CaraMunich in addition to the Extra Dark Crystal.