The only dyno I know of on the subject of the upgrade turbos on gasoline is Kruppa's build which can be read
here.
You'll notice a dyno on 104 octane, (caution, no mention of any SAE correction) producing 550 lb/ft of torque and 560 HP. This was on stock fuel system. The fact is that on 93, I'd agree with
@zdubyadubya as far as bang for the buck. This is where we have to look at torque, which is mean cylinder pressure and includes boost, temp, and VE. It's very unlikely to produce more than 500 lb/ft with this engine on 93. The larger turbos will result in higher HP numbers due to shifting the capacity to the right on the graph because of the less restrictive turbines, but he's right, maybe they are worth 50 HP on 93.
Now, if you raise the rev and shift limits, and use even larger turbo/turbos, you absolutely can make more HP, but study the various dyno charts on this site. Even the upgraded turbos are unable to keep up with mass airflow demands to support that level of torque/ cylinder pressure at over 4500RPM. That's basically the reason I don't think the upgrade turbos are worth the price.
If you have to run 93, there's only so much HP available. On the bright side, even at ~600 crank, you'll stomp 99% of cars out there!
