Online vs. LBS Components

There's no problem with the mix and match of components. My bike has FSA cranks, Dura-Ace derailleurs and Cane Creek brakes.

That said, have you considered used? For a little more than what you wanted to spend ($800), I bought a carbon frame with the above components. It's not a name brand (it's a Motobecane), but I like the way it rides and there was no way I was going to spend $2500 to get a similar bike new when I wasn't sure how much I was going to like riding a road bike. You should be able to find a frame in good condition with 105 components for around $1000.00, and I think most people will tell you that they prefer the ride of carbon over aluminum.

Your LBS might even be able to help you locate one. And when you take a used bike in for service vs. one they know came from Nashbar, they're probably going to be friendlier. Just a thought.

Bikes in the very low $1000 range normally come with a mix of Sora and Tiagra plus house brand components (Raleigh-Avenir, Trek-Bontrager, etc etc). There shouldn't be any 2300 at that price.

But going back to your questions: 

Buying from a store:

You get to test ride the bicycle. 

After sale minor adjustments will be free more often than not (the relationship between you and the shop is worth a few $$$)

At that price range, there is probably no significant difference between online and LBS bike specs.

Big online retailers get more discounts because they buy more from the distributors so they can be aggressive with the price.

My first bike, a Trek 1.5 had a mix of FSA, Shimano and "Bontrager approved" (FWTW) components. I have yet to extract every last bit of performance out of it.