Water in driver's footwell

[guzerone]

I recently bought a 2011 E70 Xdrive35d, got it cheap because it has a salvage title. One of the problems was that the carpet in the driver's foot well was wet. At first I assumed it was the sunroof drain but after pouring 2 gallons of died water down the sunroof drain I determined that that wasn't it. So I removed the carpet (what a PITA, seats and center console have to come out) and left it outside in the rain over night.
Turns out the water was leaking in between the firewall and the plate that holds the brake booster. There is some sort of gasket but I guess it must have failed.
I considered removing the plate and installing a new gasket but that looked like a huge job (no room to work in, bunch of wiring harnesses in the way, etc. so I applied rtv sealant on the outside (had to remove the brake booster for that) and painted it over with spray rubber.
It's been dry ever since.