OK I know this topic has been done to death. I had been flying a couple of LA 46 engines in two home brewed Twister sized profiles. I recently checked over the engines, as the models had been in store for a few years.
I noticed that the venturi diameters were different. Checking them out, I realised one was an FP40 venturi and the other was an FP20 venturi. I must have picked up an odd pair when I assembled the engines for stunt. Both have the standard OS NVA assembly.
As far as flight performance was concerned there was no apparent difference, except one was a little more heavier on fuel than the other. I seem to remember that there were differing stock set ups, some with FP20 and others with FP40 venturis, with the usual claim and counter claims.
Bothe planes were as near enough 46 ounces in flying trim and both ran  APC 12.25 x 3.75 props (I had short grass in those days). They were absolutely the same in performance as far as I could tell.
I was surprised at this result, considering the difference in venturi area. I would have expected to see some difference in behaviour. Maybe I am not a very observant pilot!

Andrew,

I use the smaller venturi on all my la 46 engines on anything under 50 oz.  I have seen the difference in pulling power on a plane over 50 oz. using the larger venturi. Larger venturi more fuel, yes

You're probably running the engines different degrees of rich. Stunt settings are almost always that. The over rich setting cools the engine and keeps it chugging along at a well regulated speed. That's the key. An engine that does a predictable steady run. Speeding up a tad, when the nose points up. Ideally slowing down some when the nose points down. Anytime the needle is set rich, raw unburned fuel goes out the exhaust. With a 2-stroke, even when set lean, there is a significant amount of  wasted unburned fuel. Not a very efficient system for extracting power, at least from the stand point of extracting maximum power from a given quantity of fuel. With the smaller venturi you're running a bit leaner to maintain a given prop speed. LA46s can be run that way and still provide a well regulated speed. The run is slightly different than the run with a bigger venturi set to run richer mixture which consumes more fuel. The difference in run quality might not be that noticeable when observing the flight. Depending upon the prop/weight of the plane/drag and other unknown factors, the run might be even more stunt friendly with the smaller spigot. The smaller venturi allows you to tune the engine to a more efficient (better mileage) range without overpowering the model, causing it to fly faster than you like.