can an RS4 head bolt up to an S4 block? I don't know, but I don't see why not. How much would that cost? I'm pretty sure quite a bit. To make an S4 the same level as the RS4 in terms of power and RELIABILTY... is all it take is $$$$. But isn't that what you are talking about? You said that with less money, you can make an S4 perform as fast as the RS4. Agreed, you can make more power for less money than a stock RS4 but for how long?
An S4 engine making 380hp is not going to last as long as a STOCK RS4 engine making the same amount. That is my argument for drooling over an RS4. Lets not even talk about the body, suspension, interior.

BTW, the RS4 recaros cost over 2K each. That's already 4K. The body is another 4-5K not counting shop time which should run another 4-5K. That's 15K. So we are half way to the price difference b/w and S4 and the RS4 without any engine work.