A viscous diff wont fully lock, it needs to be warmed up before it will work. Helical will lock straight off, but both are a good upgrade from an open diff!
From what I've heard the auto diff have a lower ratio than the manual diff which woiuld improve acceleration but may decrease top speed slightly..
A viscous diff will lock cold or hot and once its worn out it will hardly lock at all you will just wear it out quicker by skidding it up cold, a helical will never lock fully or be open fully,
A viscous LSD is driven by a viscous fluid. The two halfshafts aren't directly connected, instead they have fan-type things inside this viscous fluid, when the two wheels are spinning the same speed, both the fans and the fluid are all spinning together. When one wheel starts to spin faster than the other, it creates an imbalance in the LSD, the slower wheel wants to speed up and the faster wheel wants to slow down because of the fluid. On a hard launch you won't be able to tell any different between a car with an open differential and one with a viscous lsd until one wheel breaks loose, on the viscous car the power will transfer to the wheel that isn't spinning as fast, and on the open differential that one wheel will just keep on spinning how it wishes.
A Helical uses a series of helical gears around two central gears, simply put it means you are always getting drive on both, just not 50 / 50 split, it varies the power to either side depending on application which is why its so good for things like road or targa use, but kinda crap at drift. And no you cant shim a HLSD as it does not use clutch packs or viscous fans to achieve lock, its just one hell of a hard arse solid diff, used in the GTR, Supra and a whole lot of other cars and a lot of off roaders.
If you are going to put the viscous diff into your spec S then i would highly recommend you just put the head in and keep the ring gear from your open so that you don’t lose your current ratio, while you are at it, shim it up tight as so it locks quickly and you don’t get too much axle tramp.