While I haven't used a Cometic gasket on a 919, I have used them on a variety of other engines and they without exception worked wonderfully. Basically they are the same form as the stock gasket -- a nitrile rubber coated aluminum center plate is sandwhiched between two stainless steel sheets that are coated with Viton rubber on the surfaces contacting the head and block.
The best thing about these gaskets is they don't deteriorate with use, unlike the punched plate covered with gasket material which tend to get torn up by differential thermal expansion of the head and block. The Cometic gasket allows for this by sliding the cover plates over the center plate, mitigating internal tearing. In fact this type of gasket is generally deemed to be "blow proof" it's so tough.
If you are going to bump the compression without using the services of a machine shop this is the best way to go about it.
Keep in mind that a thinner head gasket will slightly retard the cam timing which will narrow the powerband a little, usually compensated by the increased compression ratio.
Oh, and no more using regular gas!
If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
------- Rob --------