|I don't know of any published studies for any car using a 907 engine, but there are a few reliable articles (and a lot of hype, speculation, and unverifiable self-serving claims) scattered about on the internet. My comments below are based on articles and other postings I consider reliable in whole or part, on my own observations, and on informed speculation.
In the JH, the stock filter arrangement is a bit restrictive compared to the dual pancake style filters from K&N and elsewhere. The pancake filters supposedly provide a noticeable power gain, but I don't know how much, nor if they're the most cost-effective approach except when modifying an already-built engine. (Disclaimer: my car has K&N filters courtesy of a PO.)
The GT and some Lotus cars used a different intake plenum with a front-facing inlet port that in the GT's case was ducted to the radiator bulkhead. The 'ram air' effect this arrangement provides is probably trivial at speeds below 60 mph. The long corrugated inlet hose is quite restrictive but ducting with a smooth interior might be arranged. The use of cooler exterior air might increase peak power a modest amount. I'd guess that the arrangement might increase peak power by 3-5 hp, best case. This might translate to a top speed that's 1 or 2 mph higher. IMHO this is of very little use in a street car.
The use of air inlet horns can usually improve an engine's air flow. It's my understanding that for cars with Strombergs, the longer horns are beneficial in the upper mid-range (say 4500-5500 rpm) without hurting peak power much, but that the shorter horns are of little benefit for a street engine. I'd want to ensure adequate clearance between any air horn and the plenum or filter assembly in which it was to be used, before spending my money.
Possibly a useful and simple arrangement would be a conical paper air filter rigidly mounted to the hose port of a stock JH plenum, combined with a rather large-diameter hose running from the front of the car, that would apply cooler air to the vicinity of the filter when the car was in motion. There wouldn't be any significant 'ram air' effect, but there would be at least some outside air available when the car was moving, and the original crankcase ventilation scheme could be retained. This might even be a good compromise for really cold environments, as the engine would receive warmer underhood air when the car was at rest and carburetor icing was more likely to occur.