A spool is basically the antithesis of a differential. It preents the torque split from changing at all. Good for going in a straight line, not so good for when you want to turn.

Quaife makes a couple of dog engagement 6-speed sequential boxes for FWD applications. I'm not sure if they only come with one bellhousing size or if they're adaptable to be used with various motors. You might want to give them a ring. Also, they come with a turque sensing gearset differential installed.