Now that I have had time to ride around and play with the exhausts I have found just one flaw with the design. As most ST owners know, the swinging arm is wider on the left side of the bike than it is on the right side, this is to accommodate the chain and sprocket.
GPR have done a fantastic job on the fabrication and welding of the parts, but do not seem to have realised that the two link pipes need to be different profiles. The right side is fine, but the left needs to be put under stress to get it to fit and still clear the swinging arm. The measurements that I have made were done without the pannier racks fitted and they would make the situation worse as they lower the end cans and will cause the cans to foul the swinging arm.
Here is the link pipe fitted to the bike and attached with the spring to the bikes headers, as can be seen, there is only 1 cm clearance without the end can fitted.
The solution is to have a different link pipe on the left hand side to the one on the right. Rather than trying to make complex bends in that pipe, the solution is to add 25 mm of length to the link where it attached to the header. That would bring the whole assembly outwards by 25mm as shown in the next picture.
GPR are made in Italy and I thought that I should bring this problem to their attention, but they do not publish an email address so I will have to use their contact form on their website and give them a link (pun) to this post. In the mean time I will attempt to get a 25 mm piece of stainless steel and make an extension myself, I might be able to get a local welder to attach it but I am not sure yet.
I will keep the blog posted on any response from GPR, but at this time I would not recommend a GPR system for a Ducati ST.