You can open the ecu box to see if anything has been written on the ecu itself... If it's been flashed some companies will write the tune on the outside of the ecu housing. It would be good to check that.
When you say that it shows the right road speed (not via GPS), are you saying it doesn't match GPS? Or just that you've confirmed via GPS, but by police "your speed is" signs or something like that... Not that it matters much in this case, more curious bc of the wording.
When does it taper out? If it's after 5500 or so rpm, that's extremely normal. What obd diagnostic tool did you use? I'm assuming that it's a 2001 a4 with a 1.8t in it? 2001 has a MAP sensor in it that actually reads the amounted of boost at the intercooler. If you are able to read specific measuring blocks, or can get ahold of someone with a vagcom, you can log block 115. It has 4 sets of numbers in it that reads RPM, "Last", boost value specified (in milli-bar "mBar"), boost value actual (in mBar). If the actual is substantially less than the specified boost value, you've either got a leak, or the turbo could be failing.
Another way to find out if you have a boost leak is to either purchase or make yourself a boost tester (there are a number of DIY's on it). Using that, you wet down the boost plumbing with soapy water and look/listen for air escaping (i.e. Bubbles). With our cars being 15 years old (and older in many cases) it's very likely that tubes and hoses have gotten dry and cracked.
Also, without knowing anything about the diverter valve, it'd be very reasonable that it could be defective and is a leak in the system. Most (if not all) aftermarket DVs can be opened and (should be) greased periodically. So it wouldn't hurt for you to take it off and see about opening it up. Also a very common place for a leak is the hose that goes from the manifold to the top of the DV.
As for consistently getting 9-10psi, it is very possible that there is a leak, if hoses are dry and brittle, cracks may not open enough until it reaches 9-10ish psi... Which is why a boost leak tester is a good idea. Using it you start at a steady 5psi and look for leaks. Any leaks at 5psi are going to be substantial. From there you slowly increase the air pressure until you're at around 20-25psi... In your case since 18psi is what you're expecting, you'd definitely want to test at least to 20psi.
While noises while rev'ing can be aggravating, what I'd really be concerned about is whether you hear any chattering during wide open throttle (WOT), engine braking just after having it WOT, or at any specific boost level while driving part throttle.
If you're capable, if recommend unbolting the downpipe from the cat and inspecting the gasket. It is a pain to get to, but gaskets are cheap. Also, if you uninstall the catalytic converter (assuming you don't have a test pipe) you can visually inspect the wastegate with a flashlight and a mirror. It's not the most convenient thing to do, but it's a tad less effort than taking the whole turbo off...
Personally, I'd be surprised if you're getting 205-215hp and 240-250tq while only reaching 10psi. That is unless the turbo is not a ko3s, if it were something like a Garrett eliminator turbo, GTRS or something of the like, that would make a bit more sense. It would also fall in line with some of the kits APR sells, since you mentioned it.
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