Mpfi has been a proven power loser. The reason is latent heat of vaporization. Fuel evaporates and cools the intake charge which densifies it and helps power. In an Mpfi system the fuel doesn't evaporate in the plenum, so the plenum is hotter and less dense.
Modern engines make big power these days by leveraging variable valve timing, better cylinder wall finishes, efficient oil control and because computer design and control have helped reduce the variability cylinder-to-cylinder so much that today they get away with 87octane and 10:1.
Not to mention variable intake manifold runner lengths, better flowing heads and exhaust, better performing cooling systems, tighter tolerances and improved materials. Just look at today's block castings compared to the old iron we deal with and tell me the 70s was a good time for casting (it wasn't).
Tbi systems should be able to run as well as a carb, but they don't. Manifolds all flow different and all have balance issues unless breathed on heavily by someone well versed in porting. Carbs are self balancing to an extent, because they only add fuel based on airflow conditions through the barrels, not based on some one-size-fits-all algorithm that needs to "learn" your engine.
I'm a lover of efi in the philosophical sense, but I've come to realize in the past two years that there's a reason Detroit got away from TBI in a hurry.. Mpfi brings other challenges and a need for more and better airflow in the intake manifold (and optimizing the heads for it would also require changes), but it seems to be the only real way to go if one expects to maintain the driveability of a carb and reap all the modern benefits of efi. Since most good Mpfi systems cost 2x what the tbi systems do, it really becomes tough to find any value when a well sorted carb is probably 75% the cost of a tbi system, but it's performance is likely smack dab between tbi and Mpfi..
I'd run a tbi on a 4x4 or very very mild (stock cam) motor all day long though. It makes most sense in a 4x4 (to me) since they tend to see all kinds of odd angles and steep inclines.
Just my humble opinion, of course.
