RE: Sail Range on ST 115
It might depend on your weight/height and even rider style/preference etc.
Having tried grunty 7.5/7.6m sails on the ST115, my feedback was that it wasn't all that flash; but the same board / nocam freeride 7.0m was really fun. Some other (customer) riders - lighter weights- seemed to think 7.5m was still OK for them.
To be at it's best, the ST's do need to be reasonably powered within their range sweetspot (they don't take oversized sails or fins as well as some alternates, like iS for example)- but then within the range the softer edge / bevel rail really makes the ride/speed a lot easier.
In real terms, you won't get a lot of extra practical sail size sweetspot from the C111, just a small amount - and in both boards you'll be pushing the upper sail size sweetspot, especially if you are a mid to heavy rider (?)
The speed of the Carve111 (2007) is pretty quick, but the new rocker in the 2007 ST is about as good as it gets (right off the S85); there is a new lower nose in 2007 with less swing weight plus lower angle of attack for better control and top end speed. The 2007 ST's feature a new fin model (FreerideFlow) which is more suitable to the full on nature of the ST (and typical ST riders..)
Hope this answers your questions, but let us know if you need more info..
Cheers ~ Ian