I've been toying around with different blends for a few years so I'll chime in with my two cents. First, I'd suggest reading more from the likes of Cannonfodder, another jim, and Compass Coffee and others who wrote on a similar topic. Unfortunately, I cannot find the thread! But it's out there--check around.
Read here for a ton of info about roast profiles--a critical component of successful espresso roasting:
Roasting Profiles for Espresso?
Here's a shorter thread on espresso blending: Favorite recipe for espresso blends
I've gotten into the habit--for better or worse, tho' usually for the better--of going quite heavy on the Brazilians. Since they often make such excellent SO espresso I figure that a bit of tweaking can make them even more interesting. In that regard, I usually use a 40% base of Brazilians--20% DP and 20% semi-washed or natural. I like the earthiness of a Sumatran or the viscous body of another Indonesian--add 20% of that. An Ethiopian or Kenyan will add fruitiness (20%), while a bright Central brings floral or spice (say 10%). Now, to blow it into another realm, try adding Monsooned Malabar to bring it to 100%. It was one of the weirdest things I'd had in espresso when I first tried it; now I just love it. (I've had a similar experience with Aged Sumatras, but they've got to be good or their smokiness will dominate.)
OK, now it looks like a friggin' grab-bag of beans...I mean, who on earth will taste 10% of a Central in a blend like that? Why make it so complicated? The simple answer, to my tongue, is that the espresso tastes better. I might have a hard time picking out the Central, but when I blend without it I find the espresso lacking. I use it regularly in blends now.
The other week I thought I'd try two blends, both with the above Brazilian/Sumatran base. One then finished the remaining 40% with a mild leftover Oaxacan, Australian Mtn. Top, and Greenwell Farms Kona. I wanted a really mellow, hopefully sweet espresso. On the other hand, starting with that same base, I thought I'd aim for something much more aggressive/smokey...so I finished it with equal parts Harar, Monsooned Malabar, and the '04 Aged Sumatran. The 'mellow blend' was so mellow that I think I saw the girl from Ipanema walk by. The smokey blend was much more interesting, but too aggressive. After three days of tasting I mixed the remaining half-pounds together and enjoyed some wonderfully complex, rich, chocolately espresso.
I've never found much use for robusta--the crema enhancement doesn't seem much needed when I'm starting with a good load of Brasil. I try to think of balance above all: balancing brighter flavors with sweetness, balancing spiciness with deeper body, balancing DP with washed. Looking at your list, I'd probably trade a Guatemalan for the robusta and try a Mandheling instead of the Aged Lintong. That would open up lots of room for the subtler aspects of the blend. As weird and complicated as blending can get, it's usually successful (there's only been one blend I've made in the last two years that I've cared less for). I hope that you continue to enjoy it and to report on your progress.
OH, one more note: never throw out a blend until it's three weeks old. Some of them taste like crap after a good week's rest, only to open up around day ten and stay great for a few days...or even longer.