I have only shot a handful of groups at 100 yards with my budget air rifles (all under $500 new). My most accurate, a P35-22, has given me one 200 on the 30 yard challenge and two three shot groups at 25 yards that were a single pellet wide. I've been tuning my P35-177 and P35-25 to try and get them to this level. Not sure I will get there. I have an original Avenger in 25 and need to mess with it some more.
My input based upon my limited attempts is consistent with the "it's a lot harder" camp. I think I was close to 3 inches with my 25 caliber Avenger and I was slightly over 2 inches with my P35-22. But there was non trivial wind both times so that was a factor, As you note, 177 has a worse bc so it is pushed around significantly more by the wind. I doubt you can get consistent sub MOA groups at 100 but you would have a better chance with slugs. They will move less in the wind because of the higher bc. My gun doesn't like them but some report good shooting in chinese barrels with knockouts, for instance. My P35-25 is more accurate with pellets but shoots some NSA and H&N slugs OK. I haven't tried very hard but finding a slug your gun really likes seems to be a lot harder than finding a pellet it likes, however.