It is going to be important to know the differences in the 4.0 TFSI especially if the rumors of the next generation RS5 getting a variant of this motor are true.
Let's take a look at a few dynographs of the 4.0 TFSI V8 in various Audi models:
A8 383 stock, 474 93 octane tune, 508 100 octane tune:
S8 485 stock, 573 93 octane tune, 610 100 octane tune:
S6/S7 403 stock, 478 93 octane tune, 490 100 octane tune:
RS6/RS7 496 stock, 600 93 octane tune, 652 100 octane tune:
It is no surprise the RS6/RS7 are the strongest stock but the S8 does not trail by too much. The S8 is rumored to have the same size turbos but it can not match the RS6/RS7 4.0 TFSI. The A8 puts up similar numbers to the S6/S7 although the S6/S7 go through a dual clutch transmission versus the automatic the other cars use. If AudiBoost were to guess the A8 and S6/S7 are using the same size turbos and the S8 and RS6/RS7 are using the same turbos.
AudiBoost intends to delve further into this subject in the next few weeks to get an idea of what accounts for the output differences. APR created this small chart (wish it was bigger) showing the difference in output between the automatic 4.0 TFSI cars:
What can be said as certain is that the 4.0 TFSI V8 is proving to be an excellent tuning platform and despite its displacement disadvantage versus the Porsche, Mercedes, and BMW twin turbo V8's it may prove to be the strongest. It certainly already appears it is as far as tune only potential.