I had the same problem with one side of the GT. Fortunately I have a pack of new bolts but I did not want to have to drill out the old bolt.
Even with the correct torx socket, there was now way it was going to hold and there is not enough metal to grip the end with a stilson or molegrip.
I soaked the bolt in Plus Gas for three nights in a row. I then carefully used a grinder to cut into the side of the head of the bolt, working down until it just cut through the washer and no more, to create a flat edge and leaving as much of the centre of the bolt as possible. Do the same from the other side.
You will then be able to knock out what remains of the washer which will take the tension out of the bolt.
I applied a large shifter to the flats cut into the end of the bolt and it came out fairly easily.
I should have the cut down bolt somewhere in the garage. If I find it I will post a photo.
"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