We have very similar backgrounds. I was one of three firearms instructors at my department. I was also on our SRU team for many years. Back in 1988 they sent me to Quantico for the FBI sniper school.
I would agree with the findings you mentioned. Especially in police work so called sniper shots are usually no more than 75 yards. There are exceptions of course but that seems to be the norm. Our rifles were 308's. We used a remington 700 in 308 and we also had an HBAR Colt in 223 with a scope on it.
I have shot tons of both calibers at distances out to 400 yards. Most of my practice never exceeded 200 yards. My groups at 200 yards were no different with a 223 than a 308. More punch of course but accuracy was very close to the same. Even moving out to 300 to 350 yards the 223 stayed right with the 308 without making any radical changes to the scope settings. Again that's accuracy not power. 2 inch groups at 300 yards was no problem with either one. I purchased a DPMS Lo Pro with a 16 inch Douglas barrel and it was amazingly accurate. I was using the same type Leatherwood scope that's on this rifle. I firmly believe that this rifle would serve very well in the police role whether it be as a tactical scoped model or setup as a standard entry gun with an EoTech sight. I have an EoTech 512 I will use on this rifle also. I put the scope on when I picked it up because I wanted to see how accurate it is but at times I'll change over to the EoTech for close in practice ( out to 100 yards but usually more like 25 to 50 yards).
I feel like that in an urban setting a 308 is a bit much. Since were talking head shots, if god forbid we have to shoot, I have complete confidence the 223 with a 16 inch barrel will do the job whether it's an AR platform or bolt action. I keep in touch with other officers around the country that are designated snipers for the department they work for and I have seen a trend towards using the AR15 with a heavy match barrel for the tactical(sniper) role. As far as barrel length you loose about 200 fps with a 16 inch barrel. It comes out to about 2500 fps in a 16 inch. Most of the departments were using 20 and 24 inch barrels which they are getting 2900 to 3000 fps depending on load. They are setup with a free float tube etc. and a high quality scope like a nightforce if they can afford one:-) as far as scopes go I see no need for anything more powerful than a 3x9 like the Leatherwood I use. This Leatherwood uses a cam system to adjust for load and distance. You begin by using a table of ammo types Leatherwood supplies, loosen the cams and set per the reference table. The cams are locked down with the weapon zeroed at 200 yards. Looking through the scope you see reference marks for target size both in inches and meters and adjust the scope power to match target size. That puts you right on target with the proper magnification. It can also be set to manual but I have been very pleased with the automatic ranging and tracking feature.
To sum it up I would prefer an AR15 with a 20 or 24 inch barrel, the ART scope and a bullet weight in the 65 or so grains. It's less cumbersome, has plenty of power for police use and allows for fast followup shots if needed. That and you don't send a big ol 308 through two houses behind the target.
On a side note I love to shoot the Savage 10FP in 308 out to 600 yards. We have a range here that was recently moved out to 600 yards. It's challenging and a lot of fun. The Accutrigger on the Savage is fantastic and easily adjusted from 1 1/2 pounds to 5 pounds of pull using a small tool supplied with the rifle. On this gun I use a scope that maxs at 24 power. I also put a Choate Ultimate Sniper stock on it. It's heavy but then I'm on a range not running up stairs to a rooftop or carrying it all over the place.
I'm getting a bit longwinded here but I hope the info helps. Feel free to email me at philwhite777@gmail.com if you want to talk about it some more.