I'll chime in with my experiences:
I've got the ATS intercooler kit on my truck. I originally had it hooked up to my old non-wastegated 088 kit with some custom ugly piping I made. It worked well, but I couldn't make over 5 psi with that old turbo no matter what I did. It did add a bunch of usable power while pulling- keeping the EGT's down.
Now it's hooked up to my Aurora 2000 kit, and what a difference! While the aurora kit made 12 or so PSI without the intercooler hooked up ( I ran it without the IC for a few weeks while making new adaptors, since ATS no longer made them) it still makes between 8 and 10 and the truck runs great. Can't get the fuel turned up enough with my current setup to get the EGT's over 1200º no matter what I do. Plenty of power, especially cruising unloaded at about 70mph. The truck is a joy to drive now, power-wise. Loaded, with about 7500lbs, it's still no new Powerstroke or Cummins, but it runs strong and doesn't drop too much speed on the long grades.
In my opinion, intercoolers should have been included from the start, with any turbo kit on these IDI engines. It made such a difference in usable power its hard to describe.
Ryan