As to the question of the F4H having a top end like the RS6, that question has to be weighed in the context of the other performance systems involved.  It’s almost certain that with additional engine capacity, whether through water/methanol injection, or upgraded motor components, the RS6 turbo would outperform the F4H in terms of top capacity.  But with the stock engine being a limiting factor, the F4H does seem capable of coming close to equaling RS6 levels of performance.

Where this question gets settled ultimately is on the road, with how the vehicle performs with each of these turbochargers. 

During the experience of on road logging I realized one of my reasons for abandoning the 605’s was flawed.  I had felt that the 605’s suffered a significant performance penalty as the outside temperatures rose.  This was the case, but over the course of tuning the F4H’s it became apparent that they too were heavily influenced by the ambient air temperatures.  Reviewing the results from each of these setups, along with my previous APR K04 hardware, it was evident that the K04 setup didn’t exhibit a similar sensitivity as shown below.

I’ve concluded that the relative insensitivity of the K04 turbocharger was more a product of the tuning and not the turbocharger itself.  APR did not push the K04 to a level of performance that is more common of late, and thus by operating more conservatively the system did not react as strongly to higher temperature conditions.

A result of this determination is that comparing the F4H and 605 on the road needs to be done under very similar temperature conditions.  The FATS times recorded for each setup under different temperature conditions is shown below.

It is apparent that the F4H and 605 react very similarly to changing temperature conditions with performance increasing as ambient temperature drops.

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During the process of settling upon a boost profile for the F4H that I felt comfortable with, there was effort made to operate the F4H at a higher boost level.  This result is shown below.  While I was not comfortable operating the turbo at this level of boost and airflow, there was no indication that it could not be operated at this level, or possibly higher.