A518 verse 727 transmission question

GeeWhiz I wish I had your problems. lol

I mean how much power are you making at 2800, then? Is this a racecar?
If yes it's a racecar then........I better be quiet,lol

But I'm pretty sure the A518 is a standard 2.45 low tranny.
That would make your current road gear starter 2.45x3.55=9.02
But if it was a 2.74, then it would be 9.73, about 11.8% higher.

The other idea,namely 2.45 x 2.5=6.13 This is a huge drop from 9.73! This is 6.13/9.73=68% . Your turbo is gonna have to make up the torque difference,namely 1/.68=47% more torque, at 2800, just to break even. Then there is the small matter of; will the 2.5 rear end take all that torque.
But if the tires spin all bets are off; you just spent a pile of time and money to see your ET go up in smoke.

How about taking a smaller bite; namely keep the 518 and swap in 2.94s. This, if a 2.74 low,gets you a starter of 8.06,which is kindof up the middle, and now your turbo will have to kick in just 9.73/8.06= plus 21%, more torque to break even.
Although cruising that monster engine with a final drive ratio of just 2.0, might be dicey, as 65mph will be around 1300 to 1600,heehee, depending on your tire diameter.I wonder how the 2800TC will like that..... Or the engine for that matter.

One thing I see, between say, the 3.55s and the 2.94s,is that the difference in mph at any particular rpm after the TC is tight,is also 20.7%; so, every point in the rpm band will be 20.7% further up the speedometer. And this will continue in every gear. In a NA car this is bad. Bad cuz less average HP is being put down per unit of distance.
But,I know nothing about turbo cars.