I believe you are talking about the blower size? If that is the case, then yes, a ton is left on the table as far as efficiency goes. Running <20psi on the street will sacrifice a ton of efficiency. All of the midrange power goes out the window and the blower won’t really shine until higher rpm’s such as 4500 and on. You would be better off with a 2.2L KB or a 2.3L whipple if staying under 20psi most of time. These smaller blowers hit peak efficiency at 24psi or so. IMO you won’t see a difference in how a PD blower makes power under 20psi, no matter how large the blower is.
If you’re running >20psi, you gain back the midrange power and the car would feel like a completely different animal because now it is making a lot more power faster and under the curve. Now the large blowers are just coming into their efficiency range and make the power that they should be making.
However, running >20psi on the street with these larger blowers is risky because of the power generated. Lighting up 315 drag radials at 70-80mph is not fun. There are a lot more things required at these boost levels as well, such as octane, and other upgrades required - all of which come with a nice price tag.
In summary, how the car feels according to the butt dyno at different boost levels is vast with large blowers. At low boost the car feels good pulling pretty hard up top. At high boost, it feels like a complete beast from low rpm all the way up to 7000rpm.
This also corresponds to track times. At low boost a large blower car will run the same time as a small blower car and comparative boost levels. Once you get above 23psi or so, the large blowers really show their difference in why they are better.
I agree, it all come down to your goals. It looks like Greg wants to run a lot of boost later on, so he should get a large blower, but be prepared to sacrifice a lot of potential until you run the high boost.
No sense in buying a small one when he plans on going big later on.
As far as gearing, I would definitely stay with the 3.55’s, you’ll definitely understand why when you hit the gas for the first time ;).