Holy crap, where do I start on this one. You seem to live in some back asswards universe, with physics that don't match the one I live in.
The 2.73 gear ratio wasn't actually my idea, I found the TSB off of Prosis from owners of 4.3 caprices complaining about the sluggish performance, and GM made a 2.73 ratio option available.
The gears are 2.93 right now, and I changed the rear tire size from 225/70/R15 to 235/75/R15 and the front tire size from 215/70/R15 to 225/70/R15. I figured that a larger radius would increase tire speed and the HP a little bit on the rears.
The only 94-96 TSB I see on sluggishness, suggests the replacement of the ICM. You might have to point out this horrible TSB, as it sounds like one of GM's brilliant moments.
Going from 2.93 to 2.56 on LT1 Roadmasters or Fleetwoods, makes them feel MUCH more sluggish. These cars have MUCH more torque than an L99 Caprice will. I drove a 2.56 Roadmaster for about a year. The only good thing about 2.56's, was that I could still drive it on the highway, even when 3rd and 4th gear didn't work, when I bought it. I put over 3,000 miles on it like that, and still regularly drove it on the highway at 60mph... in 2nd gear. When I put a new tranny in it, the thing crossed the 1/4 traps, in 2nd gear. Any time I drive it now, I just shake my head, at how sluggish the thing is off the line, compared to my similarly modded 95 Roadmaster with 2.93's.
Changing to a larger tire, is yet again, making the situation worse. You are not "Adding hp to the rear wheels". You are merely taking AWAY mechanical advantage. Taking away mechanical advantage, is the same as removing torque. Not ADDING hp. I have used 235/75/15's, on virtually all of my LT1 cars. 2.56, 2.93, 3.08, 3.73, 4.10. They all felt more sluggish off the line with the 29" tires, vs the smaller 27" or 26" tires. Vs 28's, you didn't notice so much, so I didn't include them.
My idea with the 58 cc throttle body and the polished plenum was to give the motor a slightly taller and larger plenum volume which would help build up more torque. I figure less resistance (via polishing) in the intake will help HP a little bit, but the total package is gonna just help out the torque for the most part. The ram induction should increase horsepower heavily at high speed which I think is gonna smoothen out the consequences of tuning for low end torque.
It just doesn't work that way.
A large amount of the Cammed and Heads and Cammed LT1's... still use a bone stock intake manifold, other than opening up the front bores to smoothly fit a 58mm TB.
The car really is jumping off the line, it's doing 0-60 mph in about 9.8 seconds on a good day, it's the 60-100 mph that is completely horrible.
I find it very... VERY hard to believe that your L99 car feels really strong off the line, then falls on it's face. The L99 has a much bigger reputation to struggle off the line, then start to come alive once you rev it. Which, going off how large the cam is compared to the Displacement, is what you would expect. The cam is identical from a Caprice L99 vs Caprice LT1. The TBI engines from the 91-93, were known for starting out strong, then falling on their face.