The big three.

I am interested to find out, if the gage increase in the wire to 1 leading from the +/- battery post to the starter, and the ground to the engine block will increase engine performance. Or is it just snake oil?

a little bit of a tricky question because it would depend on if there was an issue with the wiring it had, but keep in mind the better the wiring the better the starting speed, better the spark power which will relate to more power and less issues,

Regards Ross

As Ross said, it will ONLY apply to easier starting in marginal conditions.

If it is currently starting readily, it will make no difference except add weight and hassle.

There are other areas, though, where increasing the gauge of wiring 'may' potentially improve things, if marginal. A coomon weakness is the alternator output cable and ground, if required. Fuel pump power and ground, headlight wiring (one of my BIG bug-bears, as output is the 3rd power and a small voltage drop makes a big difference to filament types), ECUs and PDU/M if they're driving significant current drains, ignition drivers/amplifiers.

Starter motors, themselves, are frequently overlooked for basic servicing - if they get a lot of use, cleaning up the commutator (bit the brushes run against) with some 600 W7D and electrical cleaner, and a little grease/oil to the bushes and spiral spline the gear assmbly slides on, can make a big improvement in cranking speed and reduction in voltage drop. If it's a reduction/geared starter, a clean, check, and lube of the gearbox is also a good idea.