10mm difference between a GTV and a 156 wheel, but GT standard wheels are in the middle.
Hard to find spare GT alloys with the 35mm offset so I have used 156 16" multispokes on the GT every winter. Only 5.5mm offset difference so the visual difference is minor. No noticeable difference to handling other than the softer front end compared to the standard 18" alloys.
I now have a 156 as well, so the GT will be on 17" Speedline multispoke 156 wheels and the 156 will be on the other set. (I try not to use the GT when there is salt on the road but needs must at times).
"When you sit in a Focus RS or a Golf R32 itís like sitting in a commercial for Lynx aftershave. When you sit in an 147 GTA itís like sitting in a Venetianís hand-made suitcase." J.Clarkson 2003.
Current fleet: 156 Sportwagon jtd 16V (facelift) 2.4 Spider, Leon Cupra SC
Projects: Spider S4, Matra Murena 2.2