How much power?
 

What is the max power level you can expect to get out of an S5 stock turbo with an S-AFC?

This is assuming you're running stock injectors, maybe upgraded intercooler, perhaps an upgraded pump, stock fuel pressure.

Can you take it at least as far as the stock turbo will go? Or is there a limit to what you can do with a piggyback system?

Also, can any tuner set an S-AFC up, or just a specialist?

Seriously though.

Can you get at least 300 HP at the flywheel with an AFC?

Yes you can safely make 300HP with a safc with light mods. If you wanna go any higher I would suggest going standalone.