These changes were all to reduce emissions. The last variant (of which you posted a photo) took warm air from above the exhaust manifold and injected it into the air filter, but only when the engine was cold (there is a thermostatically controlled valve). This, I believe, reduced NOX emissions. If you put on K&N filters, you would, of course, gain hundreds of horsepower! (sorry, the subject of K&N claims are an old chestnut on this site). If you change the air filters to ones that have significantly different characteristics, you may have to change the carb needles, and timing to re-optimize the performance. With a dual Stromberg equipped car, the first thing you should look at is the secondary throttle plates:
[E-Type] Five-minute Strangleberg fix?
-David