marc pulled my secondaries when I had some troubles and the plenum needed to come off. he also cleaned up the injector housing porting a bit.
net result, no more HP but a noticeable difference in mid range and a bit better throttle response. real opportunity is if you pull the secondaries and open up the secondary ports in the heads.
not a good idea if you need to pass emissions. Graham told me that this is one of the reasons they are there. more port velocity, swirl, and less HC.
sure enough, with the secondaries (big cams and valves) I could previously get the car down to 98 HC, after the secondaries were removed, 140 was as clean as I could get
didn't really notice any difference in mileage. running at 14.7 stoich should be the same with one injector or two
definitely think it helps the car keep the secondary valves and injectors much cleaner as they are running all the time except for dead idle when the calibration cuts back to 1 injector as less than 1% throttle.