If you plan on replacing the heads and the exhaust manifolds then YES hybrids are the way forward. Hybrids cost nearly double that of modified K04's and wont give any better power really without the heads being done. Heads I have NO IDEA what they would cost - but a small fortune I would expect. So if you replace the heads fit Hybrids & the exhaust manifolds, you must be hitting 5 figures on the bill also into mid500HP .. then you are at a point where the S4 engine cant take the power and is likely to snap a rod melt something etc... so that means upgrading the internals etc etc... now we are talking 20k maybe - god I dont know.
Basically unless you have an RS4 engine, or an upgrated S4 bottom end & larger heads with exhaust manifold there is not much point spending the extra on hybrids.
Well that is my theory based on my resent research... this month anyway