Good points Bison. I guess the common misconception has to do with being able to run pure 93 octane only and enough boost to make some decent power, would be the only reasoning I could see with regards to the compression issue.
Well, there comes a point when the turbo can actually become a bottle neck on a larger CI motor. In other words, you wouldn't want to put a TA series turbo on a 274 and larger CI motor. This is mainly because of the super small compressor cover and how hot it will heat the charge temps when the boost is run up over 26psi. At least I would not recommend it.
I understand what you are saying about the turbo making the same hp at a lower boost on a larger CI motor, but is the rotating assembly comfortable running at that low a boost level?
Example, Let's say your 265 CI V6 with a TA-45a makes oh 650 flywheel hp at 28psi. (Just a guess for discussions sake) Now, you put that same turbo on a 305 CI V8 motor. It might make the same 650 flywheel hp at let's say 14psi. Now, is the compressor wheel comfortable running at 14psi? Maybe. Most compressor wheels have areas of flow where they do not like to be run at. This would be the area on a compressor map that is to the left of the dotted line. You would need to plot the flow on this 63mm wheels' compressor map to really know for sure if it likes 14psi or not.
Is the turbine wheel comfortable running at whatever psi it's running at? I'm not really sure. Exactly how much back pressure is being created before the inlet of the turbine housing? Now that we've added 2 more cylinders worth of exhaust flow and are using the same style and A/R turbine housing. My guess would be well past the acceptable 2:1 ratio. 2psi of back pressure for every 1psi of boost. So at 14psi of boost going into the motor, we do not want to see more than 28psi of back pressure before the inlet of the turbine housing. Which my guess would be slightly higher.
Some might argue that you would simply change the turbine housing. Yes, you can do this, but you need to make sure the turbine wheel is not too small to run on a larger CI motor. In the small V8 world, the P-trim turbine wheel is the smallest I would recommend on a single turbo setup. Twin turbo setups are a different matter.
The other things that come into play, when specing a turbo for a specific engine size is turbine housing options, turbine wheel selection and back pressure. A few Buick guys are fighting this battle as we speak. Trying to pick the optimum turbine housing A/R on a given motor size and still maintaining some sort of decent spool up. Some have run into extremely high back pressure readings and have had to step up to either a larger turbine wheel or larger A/R number. One of the biggest mistakes I've seen V8 guys make, is trying to run way too small of a turbo for their application, before doing any real homework on the subject.
Ok, I'll stop rambling on, hope some of this helps a little.
Patrick