There may not be a simple answer to your NCC question. I looked at the specs in the back of my manual. It seems that the answer, at least for my 2002 vintage trailer is that the slideout gains approximately 300# of cargo capacity in the dinette version and loses about the same in the lounge version. The Classic 34 without the slide has 1730# NCC; the lounge version has 1390 and the dinette version has 2080. You can investigate particular trailers with the specs in the back of the manuals on the Airstream site.
Another thing to watch is the tongue weight. The lounge version is 1250# and the dinette version is 1050.
We love our 30 slideout! We went from a 2001 Safari 25 to a 2002 Classic 30 slide. The additional room is only about 15 square feet, but it adds 2 feet to the width in a busy area and allows for an angled galley counter with more room.
Carefully inspect the wiper seal on the body that wipes the slide as it comes in and out. Mine was coming loose. The dealer "repaired" it but after a couple of ins and outs it was loose again. I reattached it with Gorilla Glue cyanoacrylate (super glue) and it hasn't budged.
Also check the gap between the slide and body to ensure that it is uniform all the way around. If not, have it adjusted before driving very far. Mine was not right and either before I bought it and didn't notice, or while I was towing it home the vibration cracked the skin of the slide at the fold on the top front corner. I adjusted mine myself after I got home, thanks to a thread on here. Search for "ready to tackle a slide adjustment" or something like that. Read and heed the warning about deploying the front stabilizers before moving the slide.
Folks have listed concerns about the mechanism, but when I opened up the cavity to check it out I was impressed. It seems very heavy duty and well done.
Good luck in your search.
"You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO
2002 Classic 30 Slideout
2013 Dodge 2500 Laramie 4x4 Megacab Cummins
2001 Safari 25 RB Twin (Gone, but not forgotten)
WBCCI 1322, TAC FL-39, AIR 82265