I'm drinking a Maris Otter/Cascade SMaSH right now. It's at 7.25 %, It's just ok. I think I'm finding that Cascade isn't all that great on it's own. But its got a nice toasty/nutty malt backbone to it, so I would suggest going it Maris Otter instead of plain old 2-Row. It will give you a little more character.
I wouldn't recommend making a high gravity SMASH. A Cascade/Two Row would be great, but I think you want to make it around 6-7% where the hops and grain can shine. Something like:
5 Gallon Batch
10 lbs Pale Two Row
1.0 Cascade @ 60
.5 Cascade @ 15
.5 Cascade @ 5
S-05 or Nottingham yeast
I would. I did a SMaSH with Domestic Pale Ale Malt (part toasted), Cascade and Cal Ale to about 8% and it was great stuff. Even better was an 8.2% Vienna/Spalt/2308 lager.
If 8% is the minimum, you'll probably need at least 14 lbs of malt for a 5 gallon batch. I'm finding that a 1:1 BU:GU by Rager is about right for a SMaSH ale for my taste.