It is painfully easy to tune CIS-E.

Get yourself a 0-10k linear potentiometer. Install it in line with the coolant temperature sensor. Go to a dyno and make some runs while measuring air/fuel ratio with a wideband O2 sensor. Turn the potentiometer until you find the optimum setting for power (typically 12.7:1 on these motors). Measure the resulting resistance, and replace the potentiometer with a fixed resistor of that value. People use different methods of switching this enrichment device into and out of the circuit manually or automatically based on WOT switch.

It is consistent, reliable and functional. One thing you are giving up to a programmable system is ignition mapping ability, but in the racing operation range you are typically at max timing regardless of the lower load/speed maps, making this a moot point.
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I do agree that the limited adjustments that are possible with CIS-E are easy, but as with most things in life, I'm afraid that the gains are proportional to the difficulty (i.e., not much gain for the easy work). I actually had to do as you suggest with an added potentiometer on a street car once. I put a 16V engine into an 8V GTI and had to make do with the 8V fuel computer. Fortunately, I was able to use a digital multimeter to measure the milliamp current at the fuel pressure actuator and did not have to pay for any dyno time to adjust the pot.

The point I was trying to make in my last post is: 1) the control pressure can be reduced for CIS-basic and CIS-Lambda, and the reduction in control pressure reduces the downward force on the fuel-metering plate thereby reducing intake restriction, in addition to increasing injector pressure; but 2) the force on the fuel metering plate cannot be reduced in this manner for CIS-E and CIS-E-Motronic (collectively "CIS-E"). Also, any reduction to the force on these CIS-E systems would cause a reduction in injector pressure (the opposite of the older systems). Thus, while tuning the basic offset for the fuel mixture may be easy, these CIS-E systems simply cannot be tuned to reduce intake restriction like the older ones could.