You still need 80% i think guild members for dungeons/raids for it to count towards guild experience on completion, 40 man raids i think are getting classed as 10 mans so 8 guildies needed for the achievements to complete. I could be wrong though but I know for TBC heroic we needed at least 4 guildies for the guild to benefit from it and recieve xp.
Oh and on our Heroic Ramparts run we were recieving around 130 guild rep per boss kill too.
my understanding is that guild rep and experience are currently 5x what it will be when live.
This is like the third time I've seen someone asking something like this. I think what people in this position are forgetting is that WoW, for better AND worse, is a social game. There are things which are being instituted which just won't work as a solo experience.
Frankly I do understand the drive to want to solo it, as I'm very much an independent player despite being in an active guild. I have alts of almost every profession so I can be self-sufficient when it comes to gems, inscriptions, enchants, etc.
That said, I definitely acknowledge that the game is DESIGNED to be played with other people. You aren't going to solo an end game raid alone (or at least not realistically) and the same goes for the guild leveling system. It is designed for groups so really you need to realize that to take advantage of it you will need to find a good home.
Now that said, why on earth would a guild dump you? That sounds strange and leads me to believe the OP might have other issues... I've never been kicked from a guild. I feel like someone who is irrationally thinking they are going to get kicked is probably not easy to play with. OR this poster has been considering letting people in his guild to level it only to kick them out again later once he has the achievements he wants so he can have his 1 person guild again. Either way, I don't see why the worry?