Cornwall has 11 - 16 comprehensive schools and large sixth form colleges
I work in Cornwall. This is not true. Most of Cornwall has 11-18 comprehensives of varying quality. A school can only be as good as the community it serves. The most troubled areas have the worst schools in my experience, although all are tainted by extreme behaviour problems.
Cornwall has schools with some appalling behaviour issues in my experience and that does impact on the teaching, learning and the results in the schools, even compared to the worst inner city schools.
Two areas of Cornwall, Truro and St. Austell (known locally as St. Awful) are served by 11 - 16 schools and two large general FE Colleges, not sixth form colleges. This does lead to problems and under achievement for some of the pupils transferring although the size of those places and various practices cover that statistic up somewhat.
There are two or three other schools, mainly Sir James Smiths in N Cornwall which no longer has a sixth form and Looe in East Cornwall which are 11 - 16 and pupils transfer either to a general FE or to another school with a sixth form.
I grew up around the Torbay area and went to school in Plymouth. I trained to teach in Exeter so I have a good knowledge of those schools too. KEVICCS is probably one of the better ones. But that is qualitative and subjective. How you view it depends on what you are used to (the schools you are coming from) and what you are expecting.
Torbay and Devon and Plymouth is plagued by the grammar school problem. Selection does leave most of the other schools in a sink situation, often with poor behaviour and poor ethos. This is made worse by the general level of hidden poverty and cultural factors. I cannot think of a single school I could recommend other than Colyton or Churston (both selective). On the other hand I can think of several you might want to avoid. :( 
To answer your question, I would say KEVICCS is OK, go for it, if you can get in.