Hi.
Been there done that and all... and I'm a fan.
In certain situations it works, and works very well. On the tiniest of clubs and bars, that's how I prefer the setup to be.
The downside of the wall of sound is the enormus requirements and dicipline it requires from the band
balance-wise, and especially from the vocalist(s).
The vocalist(s)
have to have perfect mic technique in order to avoid feedback and also have big pipes, whisperers and mumblers can forget about it.
The PA has to be top notch, with delicate channel strip and main EQ's. Compressors and gates help a mile too.
The engineer has to be top notch also, whether it's someone on-stage or off-stage.
In Your case, if the keys are always drowning everybody else and the "leader" has an improper mic technique, as it sounds from Your post, I'd forget it. The "normal" way is so much easier for everyone involved.
One advantage/disadvantage (depending of how You look at it

) is that the wall of sound can get very loud, very fast if the audience needs to feel the music too. That can be a problem if You value your hearing. It'll be an advantage if
You want to feel the music, but don't want to make the audience deaf.
He'll porobably push it and nag and whine about it to no end, so give it a try. If it's anywhere possible, try to find an "unimportant" gig to try it out though, I have a feeling that You guys can't pull it off succesfully.
Regards
Sam