Hiring the sub con direct 'might' save you some money. The extra that you pay for the main con/ID is for coordination and logistic. Also for accountability.

You hire sub con direct, when something goes wrong the they blame that the previous contractor didn't do it nicely. Then you will get passed around a lot. If you get a main con/ID then you just deal with one person. He will organize everything for you and if anything goes wrong you just hold him responsible.

More importantly if you get a good ID you will get a better Spatial layout. I see many DIY Reno where the extension and layout is so bad that they waste space or make some very uncomfortable space. 

You will actually save money because you will get clear directions and only do something once. No rehacking, no moving wall because after you put it up it seems too small/tight or something can't fit in there. No selecting colours or tiles that you thought looks nice in the showroom/catalog but when you put it in your house it looks like crap. Get it rite first time. Good ID will also show you artist impression before you start, then you know the out come, product you will be getting. 

And if they are good then your budget will be tighter. They will know all the scope of work that needs to be done and get costing for it. You will get minimal budget "surprises".