I'm sure if I asked some of your guildies why you wouldn't be able to down Sinestra 25 man a tier after it's relevancy with heroic Firelands gear is because you have 5+ people that would be idiots with cutters and/or couldn't make a valuable contribution to the raid. I personally prefer 25 man raiding but I wouldn't let some format like that decide solely if I want to join a guild. If I were to join a 25 man guild again I'd want to make sure we weren't carrying 5+ players. I do admit that a majority of heroic fights this tier are easier on 10 then 25 but not so much of a disparity to really make a difference, especially on Heroic Ragnaros. Last tier it was the opposite, a lot of fights were much harder on 10 man then 25 man, but again not enough to make a difference. 10 mans and 25 mans face vastly different problems in some ways and similar problems in others. Obviously it's harder to keep a good roster on a low pop server like ours as a 25 man.
If you blame your guilds failures on the format I don't think that's positively improving your guild because you're using it as a crutch to mask what your real problems are.