Jimbote, the 17 internals drop right into a 15 exhaust manifold, I learned that one from Kerma, the "Stealth 17" came in two versions: a 17 compressor wheel on a 15 shaft (they are the same size) or a 17 cartridge into said manifold. In order to make a "Stealth 17"; theses are old terms I know, please forgive me, the 15 compressor cover inlet had to be opened up very slightly and the 17 wheel would "fit". I emphasize "fit" as it would fit but the shape of the compressor wheels are not the same and there would be almost 0.040 of clearance in the compressor cover when the factory spec is 0.008-0.012. So yes it would move more air, but in theory it would create a lot more heat as it was trying to create boost and for lack of a better phrase: It wouldn't seal the compressed air from backing up into the housing from the charge tube. I hope that makes sense, if not there some links I can find and post. As far as billet wheels go there is a giant selection of them available, talk to the machinists.
I need to mention up front there are some differences between my 17/56 and a "standard" 17/56, but let me start at the beginning:
The gt1749vb is the "stock" turbo that came on PD engines
The gt1749vb is made into a 17/22 by upgrading the compressor wheel to 52mm
The gt2052v is a 20 series and is not a plug-and-play option
The gtb1756vk also a 20 series has a 56mm compressor wheel, once again not plug-and-play.
IIRC, a 20 series compressor housing can be made to fit the 15/17 exhaust manifold to also make a 17/56 and with a little elbow grease the stock rubber hose will slide onto the 20 outlet.
I hope this info will help answer your questions, and if any of it is wrong, please PM me and I will fix it.
At present, I am still running my Stage 3, 18 PSI tune for .205 nozzles.
I currently have my 17/56 turbo and .230 nozzles installed; I set my timing to "advanced but within spec" according to Vag-Com, and my IQ is set to 3.8 and she runs like a beast compared to before and I am sure after the new Stage 4 tune I will need new underpants after the test drive.
Due to the 17 cartridge in my turbo, 26 PSI will be the max for my tune If I had "real" 17/56 (with the 20 internals) 31 PSI all day long should not be a problem.
I am sure there will be those that read this thread and want to talk about boost creep and the difference in size between the turbine and compressor puts extra load on the shaft... Here is my polite request: if you have done exactly what I have done as an upgrade feel free to comment, but if you haven't please don't sabotage this thread with conjecture.
I didn't have $850.00 to buy a new turbo, and another $100.00 or so to adapt a standard VNT-17 to my stock plumbing and pay for shipping too.
If your cartridge is still good (mine was not) the cost to upgrade is minimal but you will need to talk to Ian at TLA.
In conclusion, will my 17/56 flow the same as a "real" 17/56, probably not but I already know that. I am not trying to get 230 hp out of my engine. I live at 4600' elev and I wanted a turbo I didn't have to spin to it's maximum RPM to provide sufficient boost to occasionally toast a ricer. 1200 171