I had one in the same condition - it was planted up against a wall, but had grown right over a bed full of overgrown shrubs. Everything was cut down to the ground and removed except the wisteria, which was cut right back to the wall - the branches were down to about 6 feet. It has never looked back and - never having flowered much before - is now a reliable bloomer. However - it would probably be a good idea to listen to others' advice as well, just in case I was just lucky. (For one thing, the trunk was only about 1.5" - 2" in diameter.)

I'd say go for it! I just "trimmed" and shaped our wisteria, which has got to be 8-10' in width, and was getting to be 8-10' tall!
I trim/shape that and the forsythia mixed in with it each year, but only trim about 2-3'. However, it re-flowers each year, so I don't think trimming will bother it.
I wouldn't trim too much, but you could probably make it either thinner or shorter without much problems in re-flowering.

Courtney