Swapping within series (S4 NA -> S4 Turbo) is easier because of the tiny amount of rewiring needed. It's really almost plug n' play. You just need to cut 1 wire, and lengthen a few others on the NA engine harness. Buying a TII harness is unnecessary for this kind of swap.

Now when sticking an S5 Turbo ECU & engine harness in, the hard part is adapting the existing S4 NA front harness (runs behind the dash, to the cluster, out the firewall, then up to the front bumper and along it). It runs EVERYWHERE, so removing it and replacing it w/ an S5 front harness isn't really an option. So you then get to redo every wire on the connector from the S4 front harness to the new S5 ECU. It'll be around 20 wires or so and I think the connector itself may be a different size. Again, the wiring diagrams and FSM emissions control section will tell you for sure.

If you want 300+ BHP, and not RWHP, then you can get there with a setup similar to mine. Maxing out the stock turbo (~12psi) with enough fuel and fuel control will get you there on a limited budget. To get 300+ RWHP, you would need a hybrid turbo like a BNR, and would need more fuel and even better control. This happens to be my next step, but I'm holding off for now since it'll run me around another $1k.