Doug,
I do NOT own a GT40 nor have I ever owned one. I looked at buying GT40's before I bought my Kirkham. The SPF GT40 is the best "production" GT40 out there, if you can't build one yourself, like an RCR, which is much less expensive, but excellent quality. There are more detailed and authentic GT40's above the SPF, but then you're talking about $500,000 (+/-). The SPF roller is roughly $115,000 before options.
One local GT40s member, Jim Craik, who owns an SPF, had a failed rear shock mount problem, which may have been corrected with the newer production units. He also had at that time about 28,000 miles, so he drives the wheels off his car. That's at least one issue the SPF GT40 had in its past.
Failed Shock Mount - GT40s.com
CC member Rick Muck (Mark IV) is an SPF dealer and knows these cars from top to bottom. I'm sure he'll check in here.
I'm already on the record here that I didn't buy a GT40 because I felt the ingress and egress were terrible (and I'm not a large individual, just average) and, once in the cockpit, the fact that the car has no roll up/down windows gave me a claustrophobic feel. I preferred the open cockpit of the Cobra.