Liquid propane injection systems like you envision are on the market and available. However the cost of the injectors is so high that most guys looking into propane go back to the old standby Impco mixers after the sticker shock of the liquid injection systems. But yes, you're right in that the gaseous propane fuel is a restriction, in theory, however I have never seen any documentation to find out if this is an actual real-world factor. You can also get gaseous propane injectors that inject the propane gas right at the cylinder ports. Sort of a compromise between the old mixer and the liquid system, with the drawbacks of both. I have read successful stories of blower and turbo guys using propane, I would guess the boost would more than make up for the potential air volume lost by the gaseous propane. For a n/a engine, either a purpose built propane engine where peak hp isn't the goal but cost per mile is, a mixer could work well. Another thought I had is a n/a engine, high compression that runs race gasoline on the strip and propane on the street. Although you would basically need to do propane injection because the dual fuel mixers are all far too restrictive for a high hp single carb engine.