Well GM did leave a ton of efficiency on the table with the TBI due to the lack of technology available at the time to meet the emissions requirements. The lack of an IAT sensor and un-heated 02 sensor causes the fueling to drift all over the place. The heated sensors were installed after the Y-pipe as well for a more accurate representation of what the whole engine was doing AFR wise. The single wire sensors have a ~30K mile life and fall off drastically in accuracy after that. When they fall off, they fall off in the overly rich direction causing much higher CO and HC readings and atleast a 2-3 mpg loss. The pellet box cat in addition to being overly resctrictive was only capable of reducing HCs and CO, not NOx thus EGR, richer mixtures and limp wristed timing advance. The pellet bed also has a tendency to clog and become even more restrictive or the screen material inside fails and fills the muffler core with the beads, choking it. It is fairly easy to get 3-5 mpg better out of a TBI especially an earlier one. I added long tube headers, dual in/single out high flow 3 way cat, a K&N filter and tuned a ~200K mile L03 305 years ago. Made more power at the wheels than GM rated it at the crankshaft and gained about 4 mpg. That 305 made 18 ft/lbs more torque than my stock L31 made at the wheels and only 8 hp less.
Higher fuel pressure does offer a better spray pattern and promotes better fuel atomization and distribution in the manifold. GM went to smaller injectors running at 30 psi to help this later on.
Your basically saying because of broken or worn parts lambda was much less than 1. Ok, that's not surprising.
TBI is an old design that isn't as good as current designs, no doubt, but at a steady state rolling down the road it's not going be 5 mpg off.