I paid a bit extra to be with Sky this year, almost 10% more I think, but I did that because I wanted my mods covered to their value. Admiral would only have given me standard parts back if I had an accident, and the risk wasn't worth the saving. Depends on whether you see insurance as a necessary evil or if you want it to give you protection/peace of mind I guess.
The fact that some specialist insurers can be double a mainstream shouldn't come as any surprise, as it's no difference to when you Gocomparethemoneysupermeerkat and end up with some premiums in the £8K range. Some insurers want the business and will take the risk, some don't want will price themselves out of the risk. Truth is that people driving modified sports cars have some of the highest value claims in the event of an accident, so insurers need to protect themselves against that risk.