In my opinion, been driving for 15yrs now and having my first import almost 10 yrs ago, i find that its the combination of sports car + age that decisions are made by individual cases.
When RAC insured my Silvia almost 10 yrs ago, no modification at ALL was permitted. I was 22 then and imports were rare as hell. At every renewing year, i was slowly permitted to mod my car.
Now, i am with West Frmers and they have insured all my previous Gtrs with unlimited modification provided that i explained in detail what every modification was done and its function.
So, in your case, call them up and ask to explain why installing an SAFC would warrant a cancellation and explain to them the function of the SAFC. In certain cases, you may be required to have an explaination written by an approved workshop.
However, as i have never had any policy with SGIO, they might have a clause or different policy. Very important: Read your policy again.