The FF sensor of the 5D is what it is all about. The IQ is excellent.
The latest and greatest 1.6X body at $1800 (7D) is only now just about pulling even with the venerable 5D.
If you can live with the AF system (center point is quite good, the other points are decoration) and the slow blackout and frame rate for your application then the 5D is good value. You have to spend $1800 on a 7D (which is going to depreciate faster) to get similar IQ.
No other 1.6X bodies have IQ as good as the 5D.
DXO-mark doesn't agree with you.
As you can see 5D classic beats 7D by big margin, specially in low light ISO.
Only in DR (Dynamic Range) 7D does show up some character as 4 year younger technology.
But the old classic is outstanding in IQ and of cause the Full Frame sensor is the magic trick, but also, it's like all things clicked together with this product.