I have a Viking QDII, which I dearly love, and supposedly has like like a 7 1/2" harp. But I am now quilting one that's 66 x. 74 and working on a diagonal, so it's functionally 1.4 x wider. I had to remove the light bulb so that the quilt wouldn't be damaged by being crammed up next to the light cover, which gets quite warm. I've found that batting thickness can make quite a difference, too. I really wish I had something bigger for quilting. 12" sounds like heaven.
hugs,
Charlotte