You're over thinking this. Your problem is your Carb.
Despite being a 408, an 850 is too much Carb.
And your sluggishness is typical of too much carb.
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You would be outright amazed at the pickup in your car once you switch to a healthy 750 cfm.
You may even pick up on the top end.
This 408 is running ported W2s flowing 297 @ .650 and 1 7/8 TTi headers . I have run my 800 AVS on it and found it had a bit more low end but worse everywhere else .
I could set up a 650 DP for running around town and use the 850 if and when I go to the track I guess .