At this point I think I'm oriented to a K551-S, because with this one I can keep my current air filter too. Do you think I'll notice a considerable improvement from the stock carburetor (obviusly when it worked in the right way...)?
I want to note that you will see us use DGV or DGS as a description. The reason is there is only one size in each, in other words the DGV is only available in the 32/36 bore, same applies to the 38...the only size in the DGS.
If the type choke has no bearing to the subject then we will often use these descriptions in reference to the topic.
Now I feel put on the spot, your question deserves an honest answer.
When and if the entire OEM ignition and fuel delivery systems worked as they were intended, no you would not notice an improvement. IMHO as fine as the Webers are I have never obtained the MPG that I can get with an OEM CJ.
I've often recommended the purchase or use of the DGV. Rebuilt, mounted and tuned them many times for myself and several others use, 17.7 MPG average is tops for myself. About 1 1/2 years ago I removed a DGV restoring a '82 CJ8 4.2 back to original equipment of course including the CarterBBD, it gets a shade over 20mpg, same roads, same conditions. Having another CJ8 an '81, all original equipment it gets over 20 mpg but I haven't driven it off the place since bringing it home.
Just recently I set a fresh rebuilt engine into another '82 CJ8 a daily driver and broke it in with the DGV that was mounted before the build. Then mounted a 38DGMS and get 17.1 mpg. I must say a lot more power than the DGV so maybe I'm wasting some with it...but I love the punch that the 38 will deliver.
Now with all that said you will notice an improvement with having a Weber instead of the often problematic BBD.
If you haven't already ordered the K551-S (the kit form 32/36DGEV) I recommend you give one more consideration to purchasing the K551-38 instead. I'll suggest a vendor that is willing to set you up with a kit that will include the OEM type air adapter instead of the gauze filter and what I consider a superior linkage to that which you would otherwise receive.