I agree with the stock turbo being capable of running low 12's.
Here is what I did back in the beginning with my first Turbo Buick.
Back in '95, I took my stock motor, stock turbo/stock intercooler '86 T-Type, to a best of 12.09 @ 109 with these mods.
Stock intercooler with the trash screen removed from the shroud and ported inlet and outlet necks, stock turbo with a stage 2 inlet bell, motor was stock with new valve springs and a double roller timing chain, both valve cover breathers, Kenne Bell plenum and 65mm t-body, 3" downpipe, 3" test pipe, with stock cat back, Super Turbo mufflers with turn downs, 36# Blue top injectors with a CPP Mini Mag race chip, Bosch external fuel pump with no intank, adjustable fuel pressure regulator, Magnacore 10mm wires, ATR air fuel ratio gauge, 160* t-stat, 3" MAF pipe, 9" K&N cone filter, dual rear air bags, Scanmaster, 26x10 ET Drag slicks on Pro Star wheels and VP C-16 leaded race fuel. This run was done on an E-brake lauch at 14psi and running 24psi. Motor had approximately 89K original miles and good compression.
The stock Garrett TBO-348 turbo is capable of safely supporting 370-375 flywheel hp when pushed. Some have actually gotten more out of it with alky and or nitrous. But, you run the risk of not only overspeeding the rotating assembly, but the charge temps are out of site at boost levels past 25psi.
The main reason you won't read about people taking their stock turbos deep into the low 12's is, most people don't realize that older posts from way back when, get purged from the system. And if those who have run those numbers with a stock turbo posted about it a long time ago, there's a good chance that their post is MIA now. I know pretty much all of my old posts from '98-'00 were deleted when the site crashed, as well as others that were purged.
There were a bunch of good posts that were done about low 12 second combos over the years. But you are correct, it is very hard to run low 12's with a bone stock car. As you can see with my own experience, it takes a carefully selected combo of parts to realistically run low 12's. There are some ringers out there that have done it with very few parts. Typically those are cars with very low miles (5000-15000 miles) and were never abused. The stock injectors become a serious restriction on reaching low 12's. Especially when you do the math. 28.5# injectors x 6, x 2 = 342 rough flywheel hp. When you take into consideration drivetrain losses, you'll be lucky to put down 300-320 rear wheel hp. Not too mention the weak stock fuel pump with inadequate wiring from the factory. It takes roughly 360-380 rear wheel hp to run 108-110 mph in the 1/4 mile with a full weight 3650# Turbo Buick. Obviously it takes less if the car is lighter.
The biggest things your combo needs are a intank pump with a hot wire kit, larger injectors with corresponding TT chip and a Scanmaster so you can tune it. Now, throw on some sticky tires and some high boost launches with race gas and 23psi and you should be in the low 12's. At least 110 octane with 25-26* timing in all gears under WOT. Don't forget to change the fuel filter, fresh plugs and a new O2 sensor. I'd recommend 50# Delphi injectors and start out with the fuel pressure set (hose off at idle) to 42psi. Watch for .800 - .750 on the O2 Mv at the end of the run and no more than 3* of sustained knock retard.
Hope this helps and sorry so long winded.
Patrick