Very fast lenses might sound like a good idea but, apart from brightening up the viewfinder, they may not be all that useful wide open because of how little will be in focus.

For low light in MF I usually use Portra 800 pushed two stops with my meter set at 1600 to 2000. I don't find grain to be a problem.

"Darn, when I first read about teleconverters I thought they went the opposite direction in f-stops..."
As an aside, there are behind-the lens wide angle converters that do increase the speed of the lens, in the same way that a teleconverter makes your lens slower, but they are very very rare, have low conversion factors and a five-figure price tag.