True I suppose, but with the right combo (even with bigger ports/runners, etc all around) it is possible to have a sorted setup that will spool earlier.

Of course if you're just optimizing the intake side (RS4 manifold/TB) then you just have more volume to fill.

Either way, I think 25 psi @ 3500 & 30 psi @ 3800 is a very lofty proposition.

Cool, looks like you have this all figured out then.  Seems Like this post and your question is pretty pointless if you don't want the advice of people with single turbo B5 S4s.

OK I'm going to post it here even though I posted it on AZ I'll post it all here so everyone sees it.
I've tuned quite a few single turbo setups now - 5858, 6262, 6266, S256, AGP 3.2, 6766 so I can tell you 100% the pros and cons of all the setups.

You can taper the boost down all you want as you head to redline but you will NOT get what you want.
The 5858 was NOT overspinning... this is evidenced by the many many (read 50+) variations I tried of boost vs timing.
Tapering the boost up approaching redline as the engine got away from the knock limit at higher rpm increased power. The 5858 simply does not flow enough air to behave as you desire.

Check this out - 5858 and 6262 on Montys car and the 6766 on my car. All on pump gas and meth:



Pretty cool huh?

The following is very neat, it shows the 5858 and 6262 on Montys car and Celisons FT21 frankenturbos.
This tells you a lot right here:



Again it all depends on what you want to do with the car.
If you spend 90% of your time in traffic or around town then yes a 5858 sized turbo is right for you.
If you do a lot of highway racing and spin to redline a lot then you want something bigger like a 6262 or 6266 sized turbo.
Or if you are crazy like me go with a 6466 or 6766 lol.

Turbo sizing is always a compromise. Always.

And just for the fun of it... here's a comparison of an E85 Frankenturbo car vs my car running E85 with the 6766 and the nitrous antilag on lol.
I think you can guess which is which ;-)

I really think the best thing for me to do would be to go for a ride/drive in these cars to really understand what I liked/wanted.  You guys should plan a drive down here haha

If the controller for your speed sensor can output 0-5v analog signal it can be wired into one of two rear o2 sensors and therefore made avilable as a variable in Me7 Logger. Read thread about hooking up zeitronix/WB o2 into stock ECU to give you an idea.

I don't know why all these people are saying you can't spool the turbo that fast,
nitrous can spool huge turbos really fast look at what the supra, and honda guys do.
 
I once put a gt30 something on a VW ABA 4 cyl ran standalone and TDI gears with a vr ring and pinion.
Ended up using a 100 shot to spool it, made 390 something wheel with 8v.