Here's the misconception, the gains to be had with an OBDII car is simply what's avalible with a distributor bluprint and carb rejetting. The very function of OBDII is to be tamper resistant, only gutting the box and installing a MOTEC allows adjustment of mixture and spark.
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Not to start an agrument, but that isn't completely true ith respect to OBD-II being not highly modifiable. Might be true on the BMW OBD-II ECUs, but on my Lightning I've got complete spark and fuel control (and everything else like MAF transfer function, retard, and literally large numbers of parameters, some I have no clue about) in 100 RPM increments, using a hand held tuner and PC. Modify, change, download to tuner, then to truck. It is OBDII too, no check engine lights, and smooth sailing.

A lot of how tunable the car is will be related to the aftermarket size - lots of Lightnings being modified, so lots of options available. There are lots of OBD-II tuners with fantastic control for lots of other popular "performance" cars, but none in IT that I know of. As you say, I bet there isn't this sort of control for say, a 1989 Porsche 944 (OBD wasn't even around). Or a 2000 BMW 328i.

You'd definitely benefit from a Megasquirt, Wolf, Motec, Electromotive, etc. ECU it sounds like. As will a lot of other modern IT racers working with their newer cars.