I had this exact problem and chased it in much the same way as you guys.
I ended up cleaning/replacing the I/C but more importantly, scrubbed the inlet manifold. I wrote how clogged it was in this thread:
Transmission Hose Gosh Darn It!
So far I have been over the moon

. On these last few hot days, minimal to unoticeable lag. It revs a lot smoother as well due to the boost occuring now closer to 1300-1400 rather than having to wait till 2000-2100.
I also notice that it doesn't change cogs as much under load now either, Seems to hold gears and revs and pull better.
Still on first tank post re-assembly but I am hoping for better economy as well.
Clean your I/C (or replace).
Remove and scrub the inlet manifold, don't forget to gently clean the manifold pressure sensor. I used MAF cleaner. It was caked up as well.
Cheers
Ralph
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