We use a ceiling plenum ducted down to the AC units with an grated ceiling in the hot rows to create a vaccuum effect pulling the hot air in, and a 2 foot plenum underfloor with perf tiles in the fronts of the racks to deliver the cold. It's pretty efficient. We don't get a whole lot of spillover, even without damming the rows appart. We did use blanking panels in the racks to stop the convection, but for the most part we've found that air balancing works pretty well. A few small tweaks to the AC settings will give you a fair bit of control as to how much cold air goes where without too much hassle.
We are testing out the chimney idea to separate even more, but so far I'm not loving it. Could be because the damn vendors keep trying to sell me fan controlled chimneys and I keep explaining to them that if you have a vaccuum on one side and a high pressure region on the other and you want air to go from the high pressure zone up to the vaccuum zone, you don't need a fan to do it. The laws of physics and thermodynamics still work. They don't grasp it. I dumb it down to the point of "this sucks hot air out. this blows cold air in. let the air handlers do their job." Nope. Please buy our fancy fans to do what nature will do anyways. Um, no. Bad vendor. No cookie. Stupid ASHRAE.