So...
Your joining the ranks of the GS now mate
For the battery, I got normal battery, just made sure the "+" & "-" were correct, if I remember correctly, the battery holder is adjustable
I think the GS cats are the same as the mine, if so - may cause an issue with the CO emssions. Just finished a full decat on mine and mines due in 2 wks for a MOT, will let you know what happens, I know the MOT is OK with the second removed, just not sure about the first
The standard wheels sizes are - 225/55/16 front and back but then you have the Sports models which have the larger rear alloys