I don't think you'll find any gain with a restrictor plate. Your butterfly valve is your restrictor plate. If you put something else upstream/downstream from there that forces you to open the throttle to compensate, you're not actually reducing pumping losses (if that's your goal). If your goal is to simply restrain how much power you use, you can do that with your brain & right foot
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You're heading in the right direction, looking @ your fuel log.