I am not sure how fast the express belt is.
Regular belt needs 2.5 fast inserters to fill one lane (or 2 fast + regular)
Fast belts need 5 to fill a lane
I expect express to need 10 per lane (well, provided it has no turns which I expect to degrade it's performance to Fast)
I just tested it, and ...

I wonder.. if you put belts right next to turrets and a creeper gets to it can they attack it? or are they pushed away the second they stop moving to attack?

Unfortunately, they don't. I tried putting chests all around to see if they could break them or if they would get pushed away by the belts ...

So I had this session on for 6 hours, and at some point I noticed that the outside bend of a transport belt moves faster than the inside bend. I thought it would make for some cool experimenting, and the rest is history.
Before fight:
http://i.imgur.com/2ZeTaJZ.png
After fight:
http://i.imgur.com ...