Joining 25mm 170 AMP Battery Cable

[Niall]

But surely if battery has a good ground to chassis, and engine has a good ground to chassis then there should be no problem? Never had an issue tbh.

Correct you are sir. People get slower cranking cars when they relocate the battery because of two reasons. Either their short earth strap is not on a clean enough surface (does need to be bare metal!) or the ring connector they have used isn't big enough or they haven't factored voltage drop when sizing their + cable.