With all the tweaking of guide locations, styles of placement, sizes, etc. How much actual difference does it make in casting distance, etc?

In my opinion, and assuming the same line, same blank, same weather conditions, same caster:

The less resistance imparted by the guides the further you can cast. Consequently, guides with bigger openings, slicker coatings, and greater elevation above the shaft of the blank (to prevent the line rubbing against it) will result in less resistance. Some of this can get counter productive if you use oversized guides (they add weight and look ugly) so you need jsut the right sizes for the rod weight and size. The number and placement of guides is dictated by the need to spread the weight of the fish pulling on the line over much of the rod but is offset by the fact that each guide will add weight and resistance. Small deviations in all this probably don't mean a hill of beans to the average caster. However, having used both wire snake guides and single foot ceramic lined guides I know for a fact that I can lliterally feel the incresed resistance of the snake guides and my casts didn't shoot nearly as well with them. I now own only rods with ceramic single foot guides.

Flyrods=10s of feet with high quality, properly placed guides, versus not. I can get a few feet over factory rods pretty easily with collector guide and singlefoot usage. Depending on the factory placement and guides, many times getting 10s of feet over them is not far out of reach either.